GEOLOGY (Gr. γῆ, the earth, and λόγος,
discourse), the science which treats of the structure
of the earth, and of the methods by which
its materials have been arranged. Under this
term are confounded two distinct branches
of study, the one being that of the chemical,
physical, and biological laws which have
presided over the development of the globe, and
the other the natural history of the earth as
displayed in its physical structure, its
stratigraphy, mineralogy, and palæontology. The
name of geognosy, employed by some authors,
may be very appropriately retained for the
latter, while that of geogeny may be restricted to
the first or theoretical division of geology. A
knowledge of physical geography, of the
distribution of land and water in past and present
times, and of the laws of winds, currents, and
climates, is one of the first requisites in the
study of geology. Then comes the investigation
of the various kinds of rocks, their
arrangement and structure, their succession and
relative antiquity, their chemical and mineralogical
history. The investigation of the chemical
agencies which have presided over the
formation of the various kinds of rocks and
minerals belongs to chemical geology, while
the laws which have regulated their deposition,
structure, and arrangement constitute dynamical
geology. The student finds that organic
life in past time played a part in the earth not less important than it does to-day, and the
study of the organic remains found in the
various rocky strata, and known as fossil
plants and animals, gives rise to departments
of botany and zoölogy which are sometimes
called palæobotany and palæozoölogy, but are
more generally included under the common
term of palæontology. The changes that have
taken place in the inorganic and organic world
introduce in their study considerations of time
and progress, and the science is found to he
largely of a historical character; the geologist,
as Cuvier remarked, being an antiquary
of a new order. Its historical element is
regarded by Lyell as so prominent that he
defines geology simply as “the science which
investigates the successive changes that have
taken place in the organic and inorganic kingdoms
of nature.” In the present article little
more will be attempted than to present a
general sketch of the history and progress of
geological science, a reference to some principal
objects of its pursuit, and the system of classifying
the groups of rocks generally adopted.
The history of the science as developed in
Europe is minutely traced in the familiar work
of Lyell, “Principles of Geology,” in which
the whole subject may also be most advantageously
studied.—From the earliest times the
structure of the earth has been an object of
interest to man, not merely on account of the
useful materials he obtained from its rocky
formations, but also for the curiosity awakened by
the strange objects it presented to his notice.
The south and west of Asia and much of the
country bordering the Mediterranean were
particularly favorable for directing attention to
geological phenomena. Earthquakes were
frequent, changing the relative positions of sea
and land; volcanoes were seen in operation,
adding layers of molten rock to those of sand
and mud filled with the shells of the Mediterranean;
the strata in the hills abounded in
evidences of similar collections of vestiges of
marine life far removed from access of the sea,
and yet unchanged during the period of human
observation and tradition; the Ganges and the
Nile, pouring forth their vast sedimentary
accumulations, were plainly building up the deltas
at their mouths, and the broad valleys reaching
far up their course were unmistakable productions
of the same series of operations in remote
periods. These phenomena could not escape
the attention of the philosophers among the
ancient Egyptians and Indian races; and their
influence is perceived in the strange mixtures
of correct observation and extravagant conceit
which make up their cosmogonies or universal
theories of the creation. In the first chapter
of the ordinances of Manu alternating periods
of destruction and of renovation are distinctly
recognized, extending in eternal succession
throughout the whole assemblage of locomotive
and immovable creatures, each period
comprehending a duration of many thousand
ages. The Greek schools of philosophy
recognized
these phenomena, which were clearly
enunciated by Ovid in presenting the doctrines
of Pythagoras. Remarkably free from
extravagant statements, they were applied to
prove a system of perpetual change slowly
modifying the surface of the earth. Aristotle
recognized the interchanges constantly taking
place between land and sea by the action of
running water and of earthquakes, and remarked
how little man, in the short span of his life, can
perceive of operations extending through the
eternity of time. Strabo distinctly applied the
raising up of land, not merely of small tracts,
but of continents also, by earthquake convulsions,
to account for the perplexing phenomenon
of beds of marine shells contained in the
interior of hills far distant from the sea.
Arabian philosophers of the 10th century are also
cited who entertained similar views of the
changes going on and their causes.—The Italian
philosophers in the early part of the 16th
century were the first to engage in systematic
investigations concerning the true nature of
fossil shells. Their abundance in the strata of
the sub-Apennine range could not fail to arrest
attention and excite inquiries, which were the
more perplexing from the limited time allowed
in popular belief to the past duration of the
earth, and from the general persuasion that no
great catastrophe except the Noachian deluge
could have occurred to modify its surface.
Various fanciful explanations were therefore adopted
in the spirit of the scholastic disputations,
and for three centuries argumentations were
sustained with much spirit on the questions:
first, whether fossil remains had ever belonged
to living creatures; and secondly, admitting
this, whether all the phenomena could not be
explained by the deluge of Noah. Among those
distinguished for the soundness of their views
in the commencement of this controversy are
Leonardo da Vinci, the celebrated painter, who
died in 1519, and Fracastoro, whose attention
was engaged by the multitude of curious
petrifactions which were brought to light in 1517 in
the mountains of Verona, in quarrying
materials for repairing the city. He exposed the
absurdities of the theories which referred the
petrifactions to a certain plastic force in nature
that could fashion stones into organic forms,
and showed the inadequacy of the traditional
deluge to bring together the marine fossils that
form solid strata of the earth. About this
time collections of these curiosities were made
for public museums and private cabinets; they
were deposited in the museum of the Vatican
at Rome, and that of Canceolarius at Verona
became famous for them. Descriptive
catalogues of these collections were published; and
as early as 1565 appeared one of the collection
of J. Keutman in Gesner's work De RerumFossilium, Lapidum et Gemmarum Figuris.
In 1580 Palissy was the first who dared assert
in Paris that fossil remains of testacea and fishes
had once belonged to marine animals. The
truth made but slow progress in the face of established prejudices. In 1669 Steno,
professor of anatomy at Padua, published his work
De Solido intra Solidum naturaliter Contento,
in which he proved the identity of the fossil
teeth found in Tuscany with those of living
sharks, and the close similarity of the fossil
testacea to living species; he traced their
progressive change from unaltered shells to solid
petrifactions, and recognized the distinction
between formations deposited by salt and by
fresh water, and that some were of an earlier
period than the introduction of plants and
animals upon the earth. But neither he nor Scilla,
the Sicilian painter, who in his Latin treatise
on the fossils of Calabria, illustrated by good
engravings (1670), ably maintained the organic
nature of fossil shells, ventured to refer their
occurrence in the strata to any other cause than
the Mosaic deluge. Leibnitz, the great
mathematician, in his Protogæa (1680), first proposed
the theory of the earth having originally been
a burning luminous mass, which since its creation
has been cooling down, and as it cooled
received the condensed vapors which now
compose its crust. In one stage of its formation
he believed it was covered with a universal
ocean. From these materials Leibnitz traced
two classes of primitive formations, the one by
refrigeration from igneous fusion, the other by
concretion from aqueous solution. The first
recognition of the arrangement of the earthy
materials in strata, continuous over large areas,
and resembling each other in different
countries, appears to have been by Dr. Lister, who
sent to the royal society of London in 1683 a
proposal for maps of soils or minerals. He also
believed that species had in past ages become
extinct. Dr. Robert Hooke near the close of
the 17th century prepared a “Discourse on
Earthquakes,” which contains the most
philosophical views of the time respecting the nature
of fossils and the effects of earthquakes in raising
up the bed of the sea. William Woodward
was a distinguished observer of the geological
formations of Great Britain, and perceived that
the lines of outcrop of the strata were parallel
with the ranges of the mountains. About 1695
he formed a collection of specimens, which he
systematically arranged and bequeathed to the
university of Cambridge. For this he
purchased the original specimens and drawings
of fossil shells, teeth, and corals of Scilla. But
his geological system was cramped by the
attempt to make it conform to the received
interpretation of the Scriptural account of the
creation and deluge. The Italian geologists
Vallisneri in 1721, Moro in 1740, and Generelli
in 1749, advanced the most philosophical views
yet presented respecting the fossiliferous strata,
and sustained them by original observations
made by the first two throughout Italy and
among the Alps. Moro endeavored to make
the production of strata correspond in time
to the account of the creation of the world
in six days, and hence was compelled to refer
them to volcanic ejections, which by floods, he
imagined, were distributed over the surface
of the earth and piled up in strata with
marvellous celerity. Buffon advanced views
respecting the formation and modification of
mountains and valleys by the action of water,
in his “Natural History” (1749), a portion of
which, contained in fourteen propositions, he
was required by the faculty of theology in Paris
to renounce. This he did in his next work,
accompanying the formal abandonment of what
he had written contrary to the narration of
Moses with a declaration of belief of all
contained in the Scripture about the creation, both
as to order of time and matter of fact.—Geology
did not begin to assume the rank of an important
science until its application to the practical
purposes of mining was first pointed out in the
last quarter of the 18th century by Werner,
professor of mineralogy in the school of mines
at Freiberg in Saxony. This distinguished man
attracted pupils from distant countries, and sent
them forth enthusiastic geologists and advocates
of the views he had conceived from his imperfect
observation of the geology of a small
portion of Germany. He taught the systematic
order of arrangement of the strata, adopting
nearly the same divisions that had been
proposed fifty years previously by Lehmann, a German
miner. He explained their production as
the result of precipitation from a common
menstruum or “chaotic fluid,” which he
supposed had once covered the whole surface of
the earth. As expounded by Jameson in 1808,
the first precipitates from this ocean were
chemical, and produced the crystalline rocks
which lie at the base of all the others, and
which he designated as the primitive class.
They included the granitic rocks and those
called crystalline schists, such as gneiss, mica
slate, clay slate, serpentine, &c. The second
class comprised the rocks he calls transition,
certain limestones, flinty slate, gypsum,
graywacke, and trap, most of which are probably
now included in the palæozoic formations.
They were supposed to have been formed
during the transition of the earth from its chaotic
to its habitable state, and to have been partly
chemical and partly mechanical in their origin,
and due to the action of the waves and
currents. The third class contained the rocks
denominated Flötz, because as observed in
Germany they were disposed in horizontal or flat
strata. In this were the coal formation, various
sandstones, the chalk, rock salt, gypsums,
various limestones, and certain traps. They were
supposed to have been formed while animals
and vegetables existed in numbers, and to have
been partly chemical and partly mechanical in
their origin. The fourth class contained the
alluvial rocks, those produced on the land, as
peat, sand and gravel, loam, bog iron ore, calc
tuff, &c., being understood to comprise all above
the chalk excepting the volcanic. The fifth
class comprised the volcanic rocks, the
pseudo-volcanic, and the true volcanic; the former
being the supposed products of the combustion of coal and sulphurous matters, the latter of real
volcanoes. These formations were supposed
to be systematically arranged; the later formed
either entirely covering the older, or, when
these form a central mountain mass, encircling
this, so that the “outgoings” of the strata
(meaning their upper edges or lines of outcrop)
form circles; those of the later formed groups
being successively larger. The basin and trough-shaped
deposits were also recognized, in which
the outgoings of the newer strata became
successively smaller. The strata, it was
understood, were subject to local disturbances from
portions sinking into subterranean cavities, and
members might be wanting in some localities,
but whenever present must be found in their
proper position in relation to the others.
Basalt, which in Saxony and Hesse was seen
capping the hills of stratified rocks, he inferred
must be of the same series of precipitated
formations, although many other geologists of
Werner's time had fully established the analogy
between this rock and modern lavas.
The observations of Desmarest, especially in
the district of extinct volcanoes in Auvergne,
made in 1768, are referred to by Lyell as
most clearly tracing the origin of the basalts
to the craters of the volcanoes. A new
controversy now arose, which for many years was
waged with animosity and bitterness unprecedented
in disputes of this class. Geologists
throughout Europe were divided into the two
classes of Neptunists, who advocated the
production of the rocks by aqueous deposition
alone, and Vulcanists, who attributed the origin
of many of them to the action of fire.
They were also called, from the names of their
respective leaders, Wernerians and Huttonians.
Dr. Hutton of Edinburgh had studied
geology for himself in different parts of Scotland
and England, and formed his own
conclusions, which he ably sustained. He was
the first to announce that geology had no
concern with questions as to the origin of things,
but that the true field of its investigations was
limited to the observation of phenomena and
the application of natural agencies to explain
former changes. His friend Sir James Hall
showed by actual experiment that the prismatic
structure of basalt might result in cooling from
a state of igneous fusion; and Hutton himself
found in the Grampian hills the granite branching
out in veins, which extended from the
main body through the contiguous micaceous
slates and limestone, thus indicating its having
been in a fused state at a time subsequent to
the production of Werner's primitive rocks.
This discovery soon led to questioning the
existence of any primitive class of rocks the
origin of which lay beyond the reach of the
present order of things; and the announcement
made by Hutton, “In the economy of the
world I can find no traces of a beginning, no
prospect of an end,” may well have startled
men of science and shocked the religious public
in the sensitive condition to which it had been
brought by the infidel doctrines promulgated in
the latter part of the last century, especially by
men of letters in France. The Vulcanists came
to be classed with the enemies of Scripture, the
true object of investigation was lost sight of,
and the controversy was continued with such
animosity that the party names at last became
terms of reproach, and many geologists avoided
being involved in it. Workers in the field,
however, were collecting new and valuable data
that were to give to the science a more exact
character. William Smith, a civil engineer,
prepared in 1793 a tabular view of the strata
near Bath, tracing out their continuity over
extensive areas, and recognizing them by the
fossils they contained. This method of
identification and of arranging strata in their true
positions he taught himself, and was the first
to promulgate in England. With extraordinary
perseverance he continued to prosecute his
work alone, travelling on foot over all England,
freely communicating his observations, and in
1815 he completed a geological map of the
whole country. In France the importance of
fossils as characteristic of formations was also
beginning to be appreciated. Lamarck and
Defrance earnestly engaged in the study of fossil
shells, and the former in 1802 reconstructed
the system of conchology to introduce into it
the new species collected by the latter in the
strata underlying the city of Paris. Six years
previous to this Cuvier had established the
different specific character of fossil and living
elephants, which opened to him, as he said,
views entirely new respecting the theory of the
earth, and determined him to devote himself
to the researches which occupied the remainder
of his life. In 1807 the geological society
of London was established, with the professed
object of encouraging the collection of data,
multiplying and recording observations, with
no reference to any “theories of the earth.”
Its active members completed the classification
and description of the secondary formations of
Great Britain, so well commenced by William
Smith; while at the same time the tertiary
formations were thoroughly investigated by
Cuvier, Brongniart, and others in Paris. Thus
each country contributed to the advancement
of geological science in the department connected
with its most prominent formations:
Germany in that of the lower stratified and crystalline
rocks, and especially in the mineralogical
structure of these, while in Scotland the
character of the granitic rocks had been more
particularly elucidated, in England that of the
secondary strata and their order of arrangement,
and in France the tertiary. The great
principles gradually developed by these
observations were: that the materials of the stratified
rocks were sedimentary deposits that had slowly
accumulated in the beds of ancient seas and
lakes; that each stratum represented a certain
period during which its materials were
gathered, and that this period was characterized
by its peculiar group of organized beings, the vestiges of which were buried and remained
with it as records of the condition of this
portion of the earth during this time. The piles
of strata of various kinds indicated changes in
the character of the deposits introduced,
sandstones formed from sand, alternating with shales
formed from muddy and clayey deposits, and
with calcareous strata, whose origin may have
been in marl beds or the remains of calcareous
organisms. The long succession of these strata,
in connection with the evidences of their slow
accumulation, observed in the undisturbed
condition of the fossil remains which they
contained, bore witness to long periods occupied
in the production of a single group of strata
constituting but a minor division of one of
the formations. The lapse of long periods was
also indicated by the fossils found in beds of
older date becoming constantly more and more
unlike existing species. The same localities,
too, presented in their successive beds some
that were filled with marine vestiges alone,
corallines and sea shells, in layers of such thickness
that ages must have passed while they
were quietly accumulating; and above or
below these were found other strata indicating
that the surface at another period was covered
with fresh water, the organic remains which
they contained being only of the character of
those belonging to ponds and rivers; and yet
again these localities became dry land, and
were covered with the forests of tropical
climes, and peopled with numerous strange
species of animals, whose nearest living
analogues are met with only in hot countries.
Such changes as these also plainly marked
slowly progressing revolutions, the period of
which no one could compute by years. It was
apparent that the sediments had collected as
beds of sand and clay now collect in seas and
lakes, and especially about the mouths of large
rivers; but it was only in such as were evidently
the product of the streams of the present
day that the organic vestiges were recognized
as belonging entirely to familiar species. In
these alone were discovered any relics of man
or any indications of his existence; and here
they were not wanting, for in the calcareous
strata in process of formation and filled with
recent species of shells human remains have
been found. But with the first step backward
the bones of extinct gigantic mammalia introduce
us to strange groups of animals, and no
satisfactory evidence is afforded, either in the
strata or in tradition, that man was their
contemporary. Thus in the closest connection,
geologically speaking, are we presented with
the most striking examples of other great
principles developed by geological research,
viz., the extinction of old and the introduction
of new species.—In consequence of the system
of observation and close investigation now
established, geology lost its highly speculative
character, and rapid progress continued to be
made in acquiring correct information of the
arrangement of the strata of different
countries.
While the defects of Werner's classification
were exposed, the general plan of it
was seen to be founded in nature, and attention
was directed to collecting everywhere the
materials for filling out the vertical column of
the rocks, as well as mapping them throughout
their horizontal range. In every country
some formations could be recognized, from
which as a base a local classification might
proceed to contiguous groups, and thus at last
the whole be included in one system of
classification. So the work of descriptive geology
has ever since been going on, new discoveries
continually adding to its completeness
and helping to the compilation of a perfect
system, which in this case should present a
full chart of the rocks from the lowest or
oldest to the uppermost or newest. Strata
lying in juxtaposition in one region, when
identified in another, are found to be
separated by the interpolation of a new series;
and again, in tracing out over broad areas a
group of sedimentary strata, they are found
gradually to assume new features, and even
to undergo an entire change of chemical
composition. The deposits over different parts of
the ocean's bed are found to be here sands and
gravels brought by currents, and there soft
calcareous muds, the remains of minute animal
organisms accumulated in still waters. The
organic remains as well as the mineral
character of these contemporary deposits present
wide differences. From the mode of their
formation it is evident that all stratified formations
must be of limited area, and must thin
away at their edges, presenting the shape of
lenticular sheets lapping upon each other.—In
1819 the geological society of London, through
the labors of Mr. Greenough and his friends,
published a map of England which was a great
improvement upon that of Smith. About the
same time Leopold von Buch prepared a similar
map of a large part of Germany. A
geological survey of France was ordered in 1822
by the French government, by which a
complete geological map of France was finally
constructed in 1841. M. Bronchant de Villiers,
professor in the school of mines, was appointed
to take charge of the work, and with him
were associated Élie de Beaumont and Dufrénoy.
The attention of these geologists was
first given to an examination of the strata above
the coal formation of England, where they had
been most carefully studied and particularly
described by Conybeare and Phillips in their
treatise on “The Geology of England and
Wales” (1821). The secondary strata of
Germany also were familiar to geologists; and
both countries consequently furnished important
points of reference for the arrangement
of the groups of France. The chalk formation
of Paris, the upper member of the
secondary, served as the starting point, and
proceeding from this they examined in detail the
lower strata as they appeared successively
emerging from beneath it, and identified them, as they could, with the corresponding groups
of other countries. Such is the method ever
since pursued, by which our knowledge of the
strata which make up the outer crust of the
earth has been systematically extended. The
importance of the organic remains found in
the rocks has been more and more appreciated,
and the shells constituting the chief portion
of these have been most thoroughly studied;
for while the different formations or groups of
strata may contain numerous similar beds of
limestone, sandstone, slates, and shales, not to
be distinguished by their mineral characters,
and which frequently cannot be traced to their
meeting with other known formations by which
their place or relative positions may be
determined, the fossils show no such indiscriminate
distribution. Each period was characterized
by its peculiar group of animated beings, and
if their arrangement is understood it follows
that the position of any stratum in which the
fossils are recognized must also be determined.
A single species may in some cases be peculiar
to one member of a geological formation, and
serve wherever the fossil is found to identify
the rock; but usually in different countries
their identification by fossils is dependent upon
characteristic genera and the order of succession
of their principal groups. This branch
of the subject will be more particularly treated
in the article Palæontology.—In the latter
part of the last and early part of the present
century papers upon geological subjects
occasionally appeared in the transactions of the
American philosophical society of Philadelphia,
the transactions of the American academy, and
in other scientific journals. The character
of these papers is almost exclusively descriptive.
There is, however, a theory of the earth
proposed by Franklin in the “Philosophical
Transactions” of 1793; and in vol. vi. appeared
the memorable essay of William Maclure, read
Jan. 20, 1809, entitled “Observations on the
Geology of the United States, explanatory of
a Geological Map.” The author of this paper
had undertaken a more arduous and gigantic
work even than that which was occupying
William Smith of England; it was no less than
a geological survey of the United States alone
and at his sole expense—a work which entitled
him to the appellation he has received of the
father of American geology. In this pursuit
he crossed the Alleghanies fifty times, visited
almost every state and territory in the Union,
and for years continued his labors mostly among
those who could have no appreciation of his
objects. He had visited nearly all the mining
districts of Europe, and thus was well qualified,
for one of that period, to recognize the
corresponding formations of the two continents.
He traced out the great groups of strata then
designated as the transition, secondary, and
alluvial, in their range from the St. Lawrence
to the gulf of Mexico. The tertiary, however,
he did not recognize, owing to the absence of
the chalk formation, the upper member of the
secondary, which in Europe, being largely
developed and most conspicuous, marks the strata
of more recent origin lying above it as tertiary.
He continued his explorations after this report,
and in May, 1817, presented another to the
philosophical society, accompanied by a colored
map and sections. His observations were
also extended in 1816 and 1817 to the Antilles,
and a paper upon the geology of these islands
was published in the first volume of the “Journal
of the Academy of Natural Sciences.”
Prof. Silliman of New Haven, educated to the
profession of the law, was induced by President
Dwight of Yale college to qualify himself
for the departments of natural science,
particularly chemistry; and with this view he
spent some time previous to 1806 in England
and Scotland. In Edinburgh he became
familiar with the discussions of the Wernerians
and Huttonians in that transition period, as he
styles it, between the epoch of geological
hypothesis and dreams and the era of strict
philosophical induction in which the geologists of
the present day are trained. The interest
excited by this controversy could not fail to
direct his tastes toward the new science, and he
returned to become its zealous promoter, for
half a century or more aiding to elucidate the
geology of his country, inspiring the enthusiasm
of others, and furnishing in the “American
Journal of Science” an organ for the diffusion
of scientific knowledge. At that period
(1804-'5), he says, geology was less known in
the United States than mineralogy. Most of
the rocks were without a name, except so far
as they were quarried for economical purposes,
and classification of the strata was quite
unknown. Dr. Archibald Bruce of New York
commenced in 1810 the publication of a journal
devoted principally to mineralogy and
geology, the earliest purely scientific journal
supported by original American communications.
It was well received at home and abroad, but
appeared only at wide intervals, and ended
with the fourth number. The mineralogical
collections at the principal colleges, and others
belonging to scientific men mostly in New
York, promoted inquiry and observation
concerning the geological relations of the minerals
and their distribution. The admirable treatise
on mineralogy by Prof. Parker Cleaveland,
published in 1816, fostered while it gratified
this spirit of inquiry. In 1818 the brothers
Prof. J. F. Dana and Dr. Samuel L. Dana
published a detailed report on the mineralogy and
geology of the vicinity of Boston. In the same
year was first published the “American Journal
of Science,” which has continued ever since
to be the chief periodical American recorder
of the progress of the sciences. The next year
the American geological society held its first
meeting at New Haven, where it continued to
meet annually for several years. The importance
of geological explorations, with the view
of thereby ascertaining the agricultural and
mineral capacities of large districts, was beginning to be appreciated by communities and
public bodies. In 1820 a geological survey of
the county of Albany, N. Y., was made under
the direction of the agricultural society of the
county by Prof. Amos Eaton and Dr. T. E.
Beck. Two years afterward Rensselaer and
Saratoga counties were also thus explored.
Prof. Eaton was also engaged by Gen. Stephen
Van Rensselaer to make at his expense a
geological survey of the country adjacent to the
Erie canal. The result of this was published
in 1824 in a report of 160 pp. 8vo, with a profile
section of the rock formations from the Atlantic
ocean through Massachusetts and New York
to Lake Erie, the Rev. Edward Hitchcock
furnishing many of the details through Massachusetts.
The first geological survey made by state
authority was that of North Carolina in 1824
and 1825, by Denison Olmsted. Since that
time there have been various surveys by the
different states or by the federal government,
of which we shall notice the most important
historically. Beginning at the northeast, early
surveys were made of Maine, New Hampshire,
and Rhode Island, by Dr. C. T. Jackson, in
1836-'41; of Massachusetts, by Edward Hitchcock,
in 1830-'40; of Connecticut, by J. G.
Percival and C. U. Shepard, in 1836, and of
Vermont in 1845-'6, a work which was
continued by Edward Hitchcock and his son, C.
H. Hitchcock, in 1858-'60, the latter of whom
is now (1874) engaged in a resurvey of New
Hampshire. In 1836 was commenced the survey
by H. D. Rogers and his assistants of the
state of Pennsylvania, which was not
completed till 1855. The survey of New York in
1836-'42, by Vanuxem, Emmons, Mather, and
Hall, may be said to have opened a new era
in American geology by giving a complete and
systematic classification of the palæozoic rocks
within its borders, which has served as a basis
for all subsequent work to the east of the
Rocky mountains. The description of the
organic remains of the state by Prof. James Hall
is still incomplete, but five large quarto
volumes have been published. The surveys of
Michigan in 1837-'46 by Houghton, and of the
Lake Superior region in 1847-'9 by Jackson,
and subsequently by J. D. Whitney and J. W.
Foster, served to extend our knowledge of the
palæozoic rocks to the westward. From that
time to the present systematic surveys of the
various states of the great Mississippi valley
have been or still are in progress, and have
already given us a pretty accurate knowledge
of the geology of the whole of this vast region.
The history of this work is too long for the
present occasion, and it may seem invidious to
mention names among workers in this great
field; but a prominent place should be given,
in addition to those just mentioned, to D. D.
Owen, B. F. Shumard, Swallow, J. T. Hodge,
Worthen, Newberry, Safford, E. W. Hilgard,
Cox, and Tuomey. Nor should the important
labors of Oscar Lieber in South Carolina and
of Emmons in North Carolina be forgotten,
nor the elaborate survey of Virginia by
William B. Rogers, of which only partial reports
have been published. The geology of the western
portion of our continent presents characters
widely different from that already noticed,
and is now attracting great attention. Much
important information was gathered by the
labors of W. P. Blake and J. S. Newberry in the
course of the great railroad surveys undertaken
by the national government; and the geological
work has been continued in the important
survey of the 40th parallel under Clarence
King, and that of the Rocky mountain region
by J. V. Hayden. These labors are still in
progress, as is also a geological survey of
California under J. D. Whitney, and the great
geological features of this region are being rapidly
made known. Much progress has also been
made in the study of the geology of British
North America. A geological survey of
Canada, embracing the present provinces of Ontario
and Quebec, was begun in 1842 under Sir
W. E. Logan, with whom were associated for
many years Mr. Alexander Murray and Dr. T.
Sterry Hunt. In 1870 Mr. A. R. C. Selwyn
succeeded Logan in the present Dominion of
Canada, including the British territory west
to the Pacific, the field of the survey being
thus greatly extended. The provinces of Nova
Scotia and New Brunswick were early examined
by Gesner, since which time Matthew
Bailey, Hartt, Hind, Hunt, and Dawson have
done much to develop their geology. The last
named has especially studied the carboniferous
rocks of that region. A survey of Newfoundland
is in progress under Alexander Murray.
The labors of the late Sir John Richardson,
Hector, Hind, and others, have done much to
elucidate the structure of the great region
north of Canada, until lately known as the
Hudson Bay territory.—With this brief sketch
of the progress of geological research in North
America, we may now proceed to discuss the
general principles of geological classification,
and to illustrate them by especial reference to
American geology. The great groups introduced
by Werner remain essentially unchanged,
but many alterations in nomenclature and
various subdivisions and reclassifications have
since been adopted, some of which require
notice. Besides the great distinction between
crystalline and uncrystalline rocks is that of
stratified and unstratified rocks, having reference
not to their intimate structure, but to
their geognostical relations. The stratified
rocks include all those which appear to be
arranged in beds or strata, whether crystalline
or not; and the unstratified, those which, like
granites, traps, basalts, and volcanic lavas,
occur in masses which are destitute of such
arrangement, and appear to have been forced
into their present position while in a more or
less softened or molten condition. These are
often spoken of as eruptive, irruptive, or
intrusive rocks. They are with a few exceptions
crystalline, and in certain cases are not readily distinguished from those crystalline stratified
rocks in which the bedding is ill defined, either
from having been obscure from the first or
else obliterated by subsequent crystallization.
There are strong reasons for believing that the
stratified crystalline rocks, by a process of
softening and subsequent displacement or
eruption, gave rise to the unstratified rocks with
which they are often mineralogically identical;
and hence the names of indigenous and exotic
crystallines have been proposed by Dr. Hunt
to designate respectively the stratified and the
eruptive rocks. A third class of crystalline
rocks is also to be distinguished, viz.: those
which occur as veinstones in the fissures of other
rocks, and have probably been deposited from
watery solutions. Such are the quartz and
spars which form the gangue of many metallic
ores, and a large part of the so-called granite
veins. The rocks of this third class, from their
mode of formation, are designated by Dr. Hunt
as endogenous crystallines. It is in some cases
impossible to determine from its mineralogical
characters to which of these three classes a
given crystalline rock belongs. The unstratified
crystalline or eruptive rocks include the modern
volcanic lavas, which are evidently the
products of igneous fusion, and the whole class
is therefore sometimes designated as igneous
rocks. It is supposed however that many of
these rocks, as for example the exotic granites,
have never been in a state of igneous fusion,
but have assumed a plastic condition by the
intervention of water under great pressure and at
a temperature far below that of fused lavas.
They have hence been called by some geologists
plutonic and by others hypogene rocks, the latter
name signifying rocks generated beneath,
in allusion to their obvious subterranean source.
The distinctly stratified and sedimentary
character of the great formations of crystalline
rocks, and the obvious analogies which they
present in this respect to the uncrystalline
formations, early attracted the attention of
geologists. In both occur intercalated layers of
limestones, argillites, and conglomerates; and
the question naturally arose as to the origin
of the gneisses, mica schists, diorites, serpentines,
chlorite schists, and talc schists, which
are the characteristic rocks of these crystalline
stratified formations. That the elements of
these had in some way been deposited from
water, like the beds of sand, mud, and carbonate
of lime of uncrystalline strata, seemed
obvious; and hence the conclusion that they
were once, like the latter, uncrystalline strata,
which had subsequently changed their form.
In accordance with this notion, they were
designated metamorphic strata, and this term is
by many geologists used as synonymous with
stratified crystalline rocks. It was noticed that
in some instances uncrystalline sediments had
assumed a crystalline character in the
immediate vicinity of certain erupted rocks; the
effect of heat, or more probably of the heated
solutions impregnating the last, having generated
in the midst of the contiguous sediments
crystalline mineral species. It was then possible
that a formation uncrystalline in one part of its
distribution should elsewhere become crystalline,
or in other words metamorphic; and it
was conjectured that great areas of such rocks
might be the stratigraphical equivalents of
formations which are elsewhere uncrystalline
sediments. In the Alps, for example, it was
supposed that the gneisses and other crystalline
schists were of mesozoic and even of cenozoic
age, and similar rocks in other regions were
declared to be palæozoic; till at length it seemed,
such was the extension of the doctrine of
rock metamorphism, that the sediments of any
age might assume the characters of the primitive
crystalline schists. In fact, the crystalline
schists of the Alps, the British islands, and the
Appalachians have all in turn been claimed as
altered strata of palæozoic or more recent times.
But these views have been controverted, and
it has been shown that the crystalline strata
which are now found in the Alps, superposed
upon the uncrystalline fossiliferous sediments,
are really ancient strata which were crystalline
before the deposition of the latter, and in
their normal position underlie them, but by
great foldings and inversions have been brought
to overlie them. In some instances in this
region beds of apparently crystalline rocks are
met with in which occur fossils like those of
the uncrystalline sediments. These were
regarded as further evidences of the metamorphic
process which had proceeded so far as to
develop a crystalline structure in the newer
beds, without however obliterating their
organic remains. But it has been shown that
these pseudo-crystalline rocks are really
sediments of the newer periods, made up of the
ruins of the older and truly crystalline rocks.
In many other cases, as in Wales and in eastern
North America, it is found that the broken-up
materials of the crystalline schists enter into
the composition of the oldest palæozoic schists,
which are themselves uncrystalline. While,
therefore, it is clear that the crystalline schists
were deposited from water, and, as will
subsequently be seen, under conditions which,
although chemically somewhat different from
those of later times, did not prevent the
development of organic life, it is now affirmed by
one school of geologists that the great bodies
of crystalline schists do not result from the
alteration of any known series of uncrystalline
strata; so that the division between the two
established by Werner may still be retained as a
fundamental one. This view is now sustained
by Favre of Geneva, Sterry Hunt, Gümbel,
Credner, and others; but the opposite view,
which maintains a wide-spread metamorphism
of palæozoic and more recent rocks, has been
taught by very eminent names, and is still
maintained in the principal geological text
books and treatises. The partisans of the latter
view, while asserting the comparatively recent
origin of many crystalline schists, have always admitted the existence of an underlying or
basal system of stratified crystalline rocks, which
were supposed to be anterior in their formation
to the appearance of life upon the earth, and
from the apparent absence of fossils were called
azoic rocks (signifying without life). In
accordance with this nomenclature, the formations
containing the fossil remains of plants
and animals have been divided into palæozoic,
mesozoic, and cenozoic rocks (signifying
ancient, middle, and recent life); while subsequent
discoveries, indicating that life had
already made its appearance in the so-called azoic
period, have led to the substitution of the name
eozoic (signifying the dawn of life). These
four great divisions are made the basis of the
accompanying tabular view of geological
formations. The subordinate divisions of
Cambrian, Silurian, Devonian, &c., are of local
origin, which, as will be seen, is also true of the
names of most of the formations into which
these in their turn are divided. In regard to
the palæozoic rocks, which have been most
minutely studied in Great Britain and America,
the names of the subdivisions recognized in
these countries are given side by side. For the
details of the mesozoic and cenozoic rocks,
which have been made the subject of not less
careful analysis and subdivision in Europe, the
reader is referred elsewhere. A complete table
of them is given on page 109 of Lyell's
“Student's Elements of Geology” (1871).
BRITISH SUBDIVISIONS.
AMERICAN SUBDIVISIONS, WITH REMARKS.
CENOZOIC, NEOZOIC, OR TERTIARY.
Recent
Alluvial deposits, peat bogs, &c.
Post-pliocene
Unstratified glacial drift, modified drift, &c.
Pliocene
Miocene
Eocene.
Widely distributed along the eastern and southern coasts from Massachusetts
to Texas, and from Nebraska across the continent to the Pacific.
MESOZOIC OR SECONDARY.
Cretaceous.
Upper cretaceous
Lower cretaceous or Neocomian
Occurs in New Jersey, Georgia, Mississippi, Arkansas, &c., and from Texas
and the upper Missouri in many localities westward to the Pacific.
Jurassic.
Upper, middle, and lower oölite
Lias
Widely developed in the western states in various localities from Dakota and
Kansas to the Pacific.
Triassic.
Upper, middle, and lower trias
Red sandstones of the Connecticut valley, New Jersey, Pennsylvania, the coal
fields of Richmond, Va., and Chatham, N. C.
PALÆOZOIC OR PRIMARY FOSSILIFEROUS.
Permian.
Magnesian limestone
Permian
Known in Illinois, Iowa, and Kansas.
Carboniferous.
Coal measures
Coal measures
To this horizon belong the coal formations of New Brunswick, Rhode Island, Michigan, Illinois, and the great Appalachian coal field.
Carboniferous limestone
Waverley
Millstone grit
Lower carboniferous
Devonian.
Upper, middle, and lower Devonian
Catskill.
Portage and Chemung
The Erie division of the New York series. Hence Dawson uses Erian as synonymous with Devonian.
Upper Helderberg
Schoharie and Cauda-galli.
Silurian (Sedgwick).
Upper and lower Ludlow
Oriskany
The upper Silurian of Murchison, the third fauna
of Barrande. The stratigraphical and
palæontological break at the top of the Water-lime
makes two great divisions of the American
Silurian.
Lower Helderberg
Water-lime
Onondaga or Salina
Niagara
Clinton
Oneida and Medina
Wenlock
Llandovery or May Hill
Cambrian (Sedgwick).
Upper.
Caradoc or Bala
Llandeilo
Hudson River
The lower Silurian of Murchison, or the second
fauna of Barrande.
Utica
Trenton
Middle.
Tremadoc.
Lingula flags
Chazy
These include the primordial Silurian and the
Cambrian of Murchison, the primal and auroral
of Rogers, the Taconic of Emmons, and the
Quebec group of Logan, and correspond to the
first fauna or primordial zone of Barrande.
Levis
Calciferous
Potsdam
Braintree and St. John's
——— ?
——— ?
Lower.
Menevian
Harlech
Llanberris
EOZOIC.
Primitive crystalline schists (Urschiefer)
Norian or Labrador
Above the Laurentian, and probably in the order
here given.
Montalban or White Mt
Huronian or Green Mt
Primitive gneiss (Urgneiss)
Laurentian
Dana uses the name Archæan as synonymous with Eozoic.
It should, however, be borne in mind that all
such divisions of the rocks are arbitrary and
artificial. From the mode in which sediments
have been deposited, and from the alternations of sea and land, it follows that there are breaks
in the succession of the rocks, which are often
marked by a want of conformity in the
arrangement of the successive formations. The
sea retires from an uplifted continent, the strata
become more or less disturbed, and perhaps in
the course of ages partially broken down and
swept away. When a new movement of the
earth's crust brings this region once more
beneath the sea, a new series of beds resting
horizontally upon the older formation is deposited,
and we have evidence, both from the
relations of the strata and from the changes in the
organic remains, of a break in the succession.
Yet it is clear that elsewhere in the region
occupied by the sea during this interval would
be deposited sediments which fill up the
interval. The process of deposition of
sediments in the sea has never been interrupted,
though the area of deposition has changed,
and all breaks in the succession are local and
accidental interruptions. Our divisions into
systems and groups have been based in great
part upon these interruptions, corresponding
to omitted leaves in the succession, which the
progress of investigation is now gradually
supplying, so that the record when completed will
show no breaks and no interruption either in
the deposition of strata or in the succession of
the forms of life. The disturbances or
cataclysms which in the theories of the older school
of geologists were looked upon as universal
are really local, and are dependent upon the
disturbances due to slow movements and the
transfer of the process of sedimentation to
other regions. But it is precisely where these
breaks have been noticed that geologists have
established horizons or lines of demarcation
upon which the systems of classification have
been built. From time to time we find out the
formations which in other regions correspond
to these interruptions, and serve to show the
transition from one of the periods to another.
These limits between hitherto separated
formations are designated beds of passage. It is
proposed to give a brief sketch of the successive
geological groups enumerated in the
preceding table, commencing with the lowest or
eozoic period, and to notice the principal facts
in their history, more especially as seen in
North America.—The rocks which we have
called eozoic include the crystalline strata,
which are regarded in the present state of
our knowledge as forming four great groups
marked by lithological differences. At the
base we have placed the Laurentian, which
consists in great part of granitoid gneiss, in
which, but for the interposed strata of quartzite,
crystalline limestone, &c., there would in many
parts be found small evidence of its stratified
origin. This ancient group is what is called in
Scandinavia the primitive gneiss, and
corresponds to the fundamental granite which is
often spoken of as underlying all other rocks.
It is the oldest series of rocks known, and in
North America forms a large part of the
Laurentides,
the Adirondacks, the Highlands of
the Hudson, and their continuation southward.
The thickness of this great series is
unknown, but Sir William Logan has estimated
that at least 20,000 ft. of strata belonging to it
are exposed on the Ottawa river. It there
includes three great limestone formations, which
are associated with iron ore, plumbago, and
phosphate of lime, and contain the remains
of a foraminiferous organism to which Dawson
has given the name of eozoon Canadense.
To the Laurentian succeeds what has been
named the Huronian, a group of crystalline
rocks much more schistose than the Laurentian,
and consisting of imperfect gneisses, with
micaceous, chloritic, and talcose schists, and
beds of hornblende and serpentine rocks,
associated with argillites and magnesian
limestones. This series is widely spread along
both the N. and S. shores of Lake Superior,
and the N. shore of Lake Huron, and constitutes
the Green mountain range of eastern
Canada and New England, stretching thence
northeastward into Newfoundland and
southwestward along the Appalachians. Rocks
apparently belonging to this series fringe portions
of the E. coast of New England, and are seen
in a wider development in the coast range of
southern New Brunswick. In some parts of
the Lake Superior region the Huronian rocks
are found to rest unconformably upon the
Laurentian, and to be made up in part of its
ruins, thus indicating a break between the two
series. The third great group noticed in our
table is that of the White mountains, or, as it
may be called, the Montalban series. It
consists in great part of gneisses, which,
however, are lithologically dissimilar from those
of the Laurentian, and are associated with large
bodies of highly micaceous schists, abounding
in kyanite, staurolite, andalusite, and garnet.
This series of rocks is traced from the White
mountains northeastward across the state of
Maine and southwestward throughout the
Appalachians. The facts, so far as known, seem
to show that it is newer than the Huronian,
resting unconformably upon it, and in some
places probably upon the Laurentian in the
absence of the former. The fourth group is
what has been called the Norian or
Labradorian, which consists in great part of granitoid
or gneissoid varieties of the rock called
norite, consisting chiefly of Labrador feldspar.
With this are associated gneisses, quartzites,
and crystalline limestones not unlike those of
the Laurentian. This series in various parts
of Canada and in northern New York appears
to rest unconformably on the Laurentian, and
was hence called by Sir William Logan the
upper Laurentian; but according to recent
observations by Hitchcock, it occurs in New
Hampshire, apparently overlying the White mountain
series. Dr. Sterry Hunt, who is the author
of this attempt to group and classify the eozoic
rocks, remarks: “The distribution of the
crystalline rocks of the Norian, Huronian, and Montalban series suggests that they are
remaining fragments of great formations once
widely spread over an ancient floor of granitic
(Laurentian) gneiss; but that these four series
mentioned include the whole of the stratified
crystalline rocks of North America is by no
means certain. How many more formations
may have been laid down over this region and
subsequently swept away, leaving only isolated
fragments, we may never know; but it is
probable that a careful study may establish
the existence of many besides the four series
above enumerated.” Notwithstanding the
distinction which has been drawn between
crystalline and uncrystalline rocks, there is probably
to be found somewhere a series of beds
marking the passage from these crystalline
schists to the uncrystalline sediments of the
palæozoic, although, so far as yet studied, the
oldest known strata hitherto referred to the
latter are completely uncrystalline, and rest
unconformably upon crystalline eozoic rocks.
There appears to be a close similarity between
the latter in widely separated countries, the
great series already indicated being recognized
with their typical characters in remote parts of
the globe.—The palæozoic rocks have been divided
into five great groups, sometimes called
systems; but these divisions, as already remarked,
are local, and the breaks in stratification and
in the succession of organic remains are in
some parts filled by beds of passage. As will be
seen in the table, there is some difference in the
nomenclature of the lower palæozoic rocks, a
portion of the Cambrian of Sedgwick being
included by Murchison in the Silurian. In the
present account we shall use these terms in the
sense in which they were applied by the former.
The lower portions of the palæozoic show no
evidence of terrestrial forms of life, their
vegetable remains consisting of algae, and their
animals of mollusks, corals, and crustaceans.
At the summit of the Silurian, however, fishes
and amphibians appear, while an abundant land
vegetation of acrogens and gymnosperms begins
to make its appearance. The palæozoic rocks
are of especial interest to the student of American
geology, as they form the surface of the
greater portion of the United States east of the
Rocky mountains. The succession of the
members of the palæozoic series in this country was
first clearly defined by the geological survey of
New York, which in its reports in 1842 included
under the name of the New York system
the whole of the known palæozoic rocks to the
base of the coal formation. The subdivisions
then established have since been generally
adopted in the United States, and their
relations to those recognized in Great Britain will
he seen in the table. The names Cambrian,
Silurian, and Devonian found their way into
American nomenclature some years later. For
an account of the progress of discovery in these
rocks, the reader is referred to the third part
of a paper on “The History of Cambrian and
Silurian,” by Dr. Hunt, in the “Canadian
Naturalist”
for July, 1872. The lower and
middle Cambrian is represented in the New York
series by the Potsdam sandstone, and the
calciferous sand rock, having a combined thickness
of less than 1,000 ft. To the eastward along
the confines of New England, and thence
northeastward along the base of the Green mountain
range, however, a series of 10,000 ft. or more of
sandstones, argillites, and limestones (including
the Levis formation), is regarded as the
representative of the lower and middle Cambrian,
and has received the names of the Taconic
system and the Quebec group. Still further
east, along the E. coast, in Massachusetts, New
Brunswick, and Newfoundland, are found strata
of lower Cambrian age, referred to the
Menevian of Great Britain. Between the middle
and the upper Cambrian in New York is a
break marked by a change in the fauna, and in
some localities by a want of conformity between
the strata. The Chazy limestone, which in some
places is wanting, shows the passage between
the two. The upper Cambrian is represented
by the limestones of the Trenton group, followed
by the Utica slates and the shales and
sandstones of the Hudson river group; the last
three divisions being known in Ohio as the
Cincinnati group. Succeeding this occurs the
Oneida conglomerate, followed by the Medina
sandstone rocks, which are in part derived from
the ruins of the underlying formations, and
which mark a period of disturbance and a break
in the succession. They are succeeded by the
Clinton, Niagara, and Onondaga formations.
The latter, sometimes known as the Salina
formation, is characterized by beds of rock salt
and of gypsum, and is succeeded by the water-lime
beds, which, as well as the other strata
of this division, from the Medina sandstone
upward, consist chiefly of dolomite or magnesian
limestone. This upper part of the American
Silurian represents the deposits in a basin
separated from the open ocean, and depositing by
its gradual evaporation strata of salt and
gypsum, the strata associated with which are
almost destitute of organic remains. They attain
a considerable thickness in Ontario and in
central New York, but thin out to the eastward
and disappear before reaching the Hudson river.
To this division succeed the lower Helderberg
limestones, characterized by an abundant fauna,
and marking by their distribution a change in
the geographical conditions of the region, by
which a deposit of marine limestone was spread
alike over all the preëxisting rocks, to the
eastward, resting unconformably upon the
Cambrian and the eozoic rocks, and attaining in
eastern Canada a thickness of 2,000 ft. or more,
where it is overlaid by a great series of
sandstones, representing the Oriskany and the
subsequent Devonian. This, in the New York
series, is marked by but a small amount of
sandstones, followed by the corniferous
limestone and the Hamilton group, which together
make up the upper Helderberg, and are
succeeded by a series of sandstones, the whole constituting the Erie division of the New York
series, the equivalent of the English Devonian
or old red sandstone, and characterized by an
abundant terrestrial fauna, the precursor of
that of the carboniferous series, into which it
passes by such transitions that it is a matter
of discussion where to draw the line. The
carboniferous series is so named because it is
the earliest and most important coal-bearing
series of strata, and includes great beds of fossil
fuel, interstratified with sandstones and shales.
At the base of the carboniferous in Michigan,
Pennsylvania, and western Virginia, and also in
Nova Scotia and New Brunswick, deposits of
gypsum and salt are met with. In the western
part of its distribution, toward the Mississippi,
the carboniferous formation includes great
thicknesses of marine limestone, which are
wanting in the east. Overlying the carboniferous
in Kansas and Iowa are beds which are
the equivalent of the magnesian limestones of
the north of England, and of the rocks called
Permian in Russia. They are regarded as the
summit of the palæozoic series.—The palæozoic
rocks correspond to the transition rocks of
Werner, to the lower part of which the name
of the graywacke series was very generally
given until the labors of Sedgwick and
Murchison classified them and established the great
divisions of Cambrian, Silurian, and Devonian.
The thickness of these groups varies greatly
in different parts of their distribution. Thus,
while the entire palæozoic series in Pennsylvania
is estimated at 40,000 ft., it is reduced to
4,000 in the valley of the Mississippi. This
is due to the fact that the great sandstones,
apparently derived from the erosion of rocks
to the eastward, thin out in the opposite
direction. In a similar manner the Cambrian
and Silurian rocks, which attain in Great Britain
a thickness of 30,000 ft., are represented
by less than 2,000 ft. in Scandinavia.—Under
the name of mesozoic or secondary rocks are
included the triassic, Jurassic, and cretaceous
series. The former has received its name
from the threefold division of it in Europe into
sandstones, overlaid by fossiliferous limestones,
which are succeeded by sandstones and shales.
At the base of the trias in the Tyrol, at St.
Cassian and Haltstadt, occurs a series of
fossiliferous beds in which the characteristic animal
remains of the trias are found mingled with
those of the palæozoic, thus showing a passage
between the palæozoic and the mesozoic rocks.
The trias, both in England and on the continent
of Europe, is characterized by beds of rock
salt and gypsum, like the Silurian and the lower
carboniferous in North America. The
sandstones of the trias in England are often red,
and constitute what is there named the new
red sandstone. The same name is applied to
sandstones of similar age which are found in
Prince Edward island and Nova Scotia, in the
valley of the Connecticut, and in New Jersey,
Pennsylvania, Virginia, and North Carolina.
To this series belong the coal fields of
Richmond,
Va., and Chatham, N. C. It is not
improbable that these beds may include strata
belonging to the subsequent or Jurassic period, so
named because it is greatly developed in the
Jura mountains. This includes both the lias and
the oölite of England, which two on the continent
are connected by beds of passage known
as the Koessen or Rhætic strata. The oölite of
England consists of highly fossilfferous strata,
chiefly marine, but in part fresh-water deposits,
and through the Neocomian (Neufchâtel)
beds passes into the cretaceous or chalk formation,
the upper part of which is characterized
in northern Europe by that pure uncrystalline
limestone known as the chalk, a deep-sea
deposit many hundred feet in thickness, made
up almost entirely of the remains of minute
animal organisms.—The rocks of the cenozoic
or tertiary period are closely connected with
the present time, and even in their lower
portions contain some species of fossil shells
identical with those now living. Lyell has
conveniently divided the tertiary, in ascending order,
into eocene, miocene, and pliocene; to these
are added a postpliocene division which includes
the period of glacial drift. (See Diluvium.)
The tertiary rocks attained a great thickness
in some parts of their distribution. Thus in
the Alps the miocene sandstones and conglomerates,
known as the molasse, have in parts
a thickness of more than 6,000 ft., while the
nummulitic limestone, a subdivision belonging
to the base of the tertiary, attains in the
Mediterranean basin a thickness of more than
2,000 ft.—We have already spoken of the trias
of the eastern part of North America. The
cretaceous is also represented in New Jersey and
along the southern border of the palæozoic
from Georgia to Tennessee. Triassic, Jurassic,
and cretaceous rocks are also widely spread
between the Mississippi and the Rocky
mountains, from Texas to Dakota, and westward
over large areas to the Pacific coast. Deposits
like the English chalk are unknown in this
formation in North America. Tertiary rocks of
various ages skirt the Atlantic coast from the
Rio Grande to New Jersey, and are even met
with off the coast of Massachusetts. They
stretch from the gulf of Mexico to Kentucky,
and like the mesozoic rocks occupy large areas
to the westward, where on the Pacific coast
they attain great thickness.—The succession of
organic life in these various groups constitutes
a study by itself, which will be considered
under the head of Palæontology. The palæozoic
age is preeminently the period of mollusks,
corals, and crustaceans, the most important class
of which last in the early times were the
trilobites, which appear in their greatest development
in the Cambrian and Silurian, and die out
in the carboniferous. Fishes, the earliest
representatives of vertebrate life, make their
appearance near the summit of the Silurian, and
abound in the upper palæozoic; reptiles first
appear in the carboniferous, and reach their greatest
development in the mesozoic, in which reptilian forms of immense dimensions, and having
curious resemblances to birds, are met with;
while the birds themselves, which then first
appeared, had remarkable reptilian affinities.
The earliest evidences of mammals appear in
the trias; throughout the mesozoic they were
insignificant in size, and chiefly marsupial. In
the eocene and miocene divisions of the tertiary
we find the greatest development of
mammalian forms. The deposits of these strata
to the west of the Mississippi have within the
last few years afforded a great number of
remarkable species of mammals, which have
been described by Leidy, Marsh, and Cope.
The flora of the tertiary period is not less
remarkable than its fauna. The geographical
and climatic conditions of the northern
hemisphere were then widely different from those
of the present day. Not only over Europe, but
in North America, and northward as far as
Greenland and Spitzbergen, a mild and equable
climate prevailed, and the abundant plant
remains preserved in the tertiary beds of those
arctic regions show a luxuriant vegetation like
that of the warmer parts of the temperate
zone of to-day. This condition of things had
been of long continuance; for in western
America great beds of coal or lignite are
found both in the cretaceous and the eocene
strata. It was continued far into the
pliocene; but as this went on, a cold climate like
that which now characterizes the northern
hemisphere prevailed, and gave rise to the
glacial phenomena which have been described
under the head of Diluvium. This change of
climate is one of the most perplexing problems
of geology. That a different distribution of
land and water and of the oceanic currents
may have contributed in some degree to this
former climatic condition of the arctic regions
is probable. Astronomical conditions connected
with changes in the eccentricity of the
earth's orbit have also been suggested as a
cause; and finally it has been supposed that a
somewhat different chemical composition of the
earth's atmosphere prevailing up to that time
may have coöperated with geographical
conditions to maintain the peculiarly mild
climate which, so far as we can judge, prevailed
throughout the arctic regions in palæozoic times,
and perhaps without interruption nearly to the
close of the tertiary.—The distribution of
metallic ores and other economic materials in the
various geological series is a point of much
interest, and demands a brief notice in this
place, although the subject is discussed more in
detail under Mineral Veins, and in the articles
on the different metals. Metallic ores are
met with both in beds interstratified with the
rocky layers and in veins cutting these. The
eozoic rocks are remarkable for their great
deposits of crystalline iron ores, of which those
of the Laurentian on Lake Champlain and
those of the Huronian on Lake Superior are
remarkable examples, as are also those of
Missouri. Similar deposits occur in the eozoic
rocks of Scandinavia and Russia. It is in
these rocks also that titanic and chromic iron
and emery occur; and to them belong graphite
and beds of iron pyrites and copper pyrites,
often associated with gold and with silver.
Oxide of tin also appears to be characteristic
of these crystalline rocks. These various ores
are found not only in contemporaneous layers,
but also in veins and beds cutting the crystalline
strata. But the metallic ores are not
confined to these more ancient rocks, for beds of
oxide and carbonate of iron are met with at
various horizons from the Cambrian up to
recent times, while under the heads of Copper
and Gold the distribution of those metals and
their ores is described. Besides these
contemporaneous deposits, veins or lodes carrying
the ores of various metals are found
cutting rocks of all ages, and are probably even
now in process of formation.—The question
of eruptive or exotic rocks has already been
briefly alluded to, but from its intimate
connection with volcanic phenomena, from which
it cannot well be separated, it is proposed to
consider the whole subject in the article
Volcano, in which connection the various
theories with regard to the nature of the earth's
interior, the sources of subterranean heat and
of ancient and modern eruptive rocks, as well
as of the gaseous products of volcanic
eruptions, will be discussed. (See also Granite.)
Under the head of Mountain will be considered
some of the most important questions of
geological dynamics, namely, those relating to
the elevation of continents, the phenomena of
denudation, and the origin of mountains. The
chemical history of the globe, or what may be
called chemical geology, will be discussed
under the titles Rocks and Water.
