A Treatise on Geology/Chapter 2
GENERAL REASONINGS CONCERNING THE SUBSTANCE OF THE GLOBE.
Chemical Data as to the Exterior Parts of the Earth.
WHAT is the nature of the mass of the globe is a
question to which chemistry and natural philosophy
furnish the only answers which our faculties can comprehend.
The nature of matter, in the abstract sense,
it is not given to man to know; but instead of this
perhaps useless, and certainly unattainable, knowledge,
we are able to discover differences among the sorts of
matter when subjected to the same conditions—differences
of weight, of hardness, of fusibility, solubility,
crystalline arrangement, and many other important
circumstances. These properties define the sort of
matter to our senses; and thus it appears that many
different compounds of matter exist in the earth. These
compounds, resolved into their elements by the processes
of chemistry, yield a certain number (fifty-five) of substances
which, under the conditions yet applied to them,
are found to be incapable of further analysis, and are
therefore called simple or elementary substances. They
are singularly diversified in weight, mode of existence
when separate, and relation to temperature and electricity.
In a free state under ordinary pressure and temperature, some (five) exist as gas; viz., hydrogen, oxygen, chlorine, fluorine, azote. Seven are non-metallic solids and liquids; viz., sulphur, phosphorus, selenium, iodine, bromine, boron, carbon.
The remainder are metallic or metalloid, and, with the exception of mercury, which is both liquid and solid within the range of terrestrial temperature at the surface, all solid.
Thirteen of these are metallic or metalloid bodies, which unite with oxygen to form the earths and alkalies, viz., sodium, potassium, lithium, aluminum, silicium, yttrium, glucinum, donarium, calcium, magnesium, titanium, strontium, barium.
Twenty-nine are what are commonly called metals; viz., manganese, zinc, iron, tin, cadmium, which decompose water at a red heat; and arsenic, antimony, copper, molybdenum, uranium, tellurium, chromium, cerium, nickel, vanadium, cobalt, lead, tungsten um, titanium, mercury, columbium, bismuth, osmium, silver, palladium, rhodium, platinum, gold, iridium, which do not decompose water.
"With the metallic and non-metallic bodies in the previous lists oxygen enters so generally into combinations which yield solid compounds, and in such large proportions, especially with the earthy and alkaline metalloids, that we may venture even to say that one half of the ponder able matter of the globe is composed of oxygen gas. The speculations, to which this conducts as to the concentration from a gaseous condition of the matter of the planetary system, seem to be in agreement with the astronomical views of Herschel and Laplace, but are perhaps beyond the range of geology, which considers not the origin of the globe, but its successive changes of condition."[1]
Table of the Proportions per cent, of Oxygen in certain abundant Earths, Minerals, and Rocks,
100 Silica = 48.4 Silicium -t 51.6 oxygen.
100 Alumina = 53.2 Aluminum + 46.8 Oxygen.
100 Magnesia = 61.4 Magnesium + 38.6 Oxygen.
100 Lime = 72 Calcium + 28 Oxygen.
100 Quartz = 48-4 Metallic base + 51-6 Oxygen.
100 Felspar = 54 Metallic bases + 46 Oxygen.
1O0 Mica = 56 Metallic bases + 44 Oxygen.
100 Granite = 52 Metallic bases + 48 Oxygen.
100 Basalt = 57 Metallic bases + 43 Oxygen.
100 Gneiss = 53 Metallic bases + 47 Oxygen.
100 Clay Slate = 54? Metallic bases + 46? Oxygen.
100 Sandstone = from 49 to 53 Metallic bases + 47 to 51 Oxygen.
100 Limestone = 52 Metallic base + 48 Oxygen.[2]
In studying the simple and various compound mineral masses occasioned by this union of oxygen with the metals and the metallic bases of earths and alkalies, the geologist labours on the same bodies as the mineralogist and the chemist, but not for the same end. To take a well known rock, granite, as an example—
"The geologist considers the circumstances under which this rock occurs in mass or in veins, with a view to determine the agencies which were concerned in its production, the period when it was produced, and other important characters. The composition of the stone is so far a matter of study for him as it helps to clear up these problems.
"To the mineralogist granite is an object of study, because it is composed of certain minerals which are characterised by certain constant properties. It is not granite that he studies, but its constituents, quartz, felspar, and mica. These minerals are investigated by their qualities of geometrical form, specific gravity, hardness, relation to light, electricity, &c. as separate objects.
"Finally, the chemist takes these separate minerals,
resolves them into their several ingredients, examines
the properties and proportions of these, and investigates
the laws of their combination."[3]
Physical Data as to the interior Constitution of the Earth.
But these oxygenised substances are only such as are found among the bodies at or near the surface of the earth; and though some of them have been elevated from considerable depths by volcanic action, the information thus acquired may not be at all applicable to the interior parts of our planet. Observation is here entirely at fault, and we must be content to remain wholly ignorant of the analytical constitution of the interior masses of the globe. We may never know what chemical or optical properties belong to it; but instead of this kind of knowledge, which, however curious, would be of little value even in theory, we have received some very important instruction from astronomy and general physics, as to the circumstances under which matter, whatever be its chemical constitution, now exists and was formerly aggregated in the interior parts of our planet.
1. Methods have been devised of measuring the attractive force of the whole globe, compared to that of some of its parts, certain mountains, for instance, and thus poising its mass against some known weight; and these methods, confirmed by astronomical inferences leave no doubt that the density and specific gravity of the globe is above five times as great as that of water at common temperatures and pressures. The average specific gravity of the principal stony masses near the surface of the earth is about 2½ times that of water; consequently, the interior parts of the earth are occupied by material substances heavier than those near the surface.
2. But it does not follow that they would be heavier if brought to the surface; for the pressure of the whole mass of the globe toward the centre must necessarily occasion a condensation of the substances, whether solid, liquid, or gaseous, therein occurring. This condensation due to mere pressure would indeed, upon all mineral compounds known to us, go so far as to augment their density much more than is requisite for the fulfilment of the condition required by the calculation. According to Leslie (as quoted by Mrs. Somerville) water would be as heavy as mercury at 362 miles below the surface of the earth, and air as heavy as water at 84 miles. Calculations of this kind, however, involve suppositions as to the continuity of the law of the density of elastic bodies being proportional to the pressure upon them; they are thus in strictness liable to objection; but the error which might arise from this cause is quite unimportant for the argument in the text. We must therefore admit that either the interior substances are naturally lighter; that they are of so different a nature as to yield but little even to the immense pressure upon them; or that their inherent elasticity is aided by some principle of expansion, which balances a part of the pressure to the centre.
3. To aid us in choosing between these cases we may call in the aid of mathematical science and astronomical observations, from which it results, 1. That the figure of the earth is an oblate spheroid, such as would be produced by revolution on its axis, provided the constituent matter of the globe were in such a state as to be allowed freely to arrange itself in obedience to the central and tangential forces concerned. 2. It is ascertained as a consequence of the theory of the moon's motions, that the interior parts of the earth are not only more dense than the exterior, but that the inner surfaces of equal density are symmetrical to the same centre and axis as the external elliptical figure.[4]
From these observations conjoined, there is no doubt that the matter of the globe, having free relative motion, was arranged under the double influence of central and tangential forces: and consequently, that the substances in the interior must be naturally at least as heavy as those near the surface under the same circumstances.
Free relative motion to the extent here required, viz. to the central parts, implies a total incoherence or fluidity of the mass of the globe. Such fluidity appears perfectly intelligible, as the effect of a general and pervading high temperature; and, perhaps, this is the only supposition which will at all meet the case. But it derives a considerable accession of probability from the fact that the earth is even now hot within; a point on which all experiments on subterranean temperature, and, perhaps, the grander phenomena of volcanoes, appear to agree; and a variety of evidence will be hereafter adduced to show that it was formerly hotter, at small depths below the surface, than it is at present.
From all this we obtain, as the most probable solution of the problem of the constitution of the interior parts of the earth, that the substances therein occurring have such analogies to those now seen near the surface, that they would have been subjected to very much greater condensation than they have suffered,—the globe would have been denser than it is,—were it not for the expansive influence of heat in the interior of the planetary mass. Whether the inner or medial parts of the substance of the globe be fluid or solid, must remain for very refined researches in physical astronomy to decide, if, indeed, evidence can be collected, on points involving the consideration of fixity or motion of the interior masses, sufficiently precise to give authority to the rigorous results of calculations applied for the purpose of testing this great question. Mr. Hopkins concludes, from the phenomena of precession and nutation, that the earth's solid crust cannot be less than one-fourth or one-fifth of its radius. (Phil. Trans. 1842.)
Mass of the Globe whence derived.
With this knowledge of the nature of the mass of the globe, the modes of combination of the several ingredients of the mass, and the properties under certain physical conditions of these ingredients existing separately, one of two conclusions must be adopted by the human mind. Either we must believe these combinations to have been original, that is to say, that the ingredients have had no separate existence and properties, till the art of chemistry found the means of disuniting and insulating them; or view the existing aggregations of matter, as results of combination of the separate elements, produced by some change of conditions. If the former view be adopted, there is no room for further discussion; if the latter, an inexhaustible source of intellectual exertion is opened, and all the mysteries of nature are subjected to the scrutiny of man.
There may be persons who view this as a matter of no importance, and would, perhaps, be content to save themselves the trouble of inquiring into the works of creation, by the indolent belief that the world was made as we see it, its complicated phenomena not produced by appropriate laws of causation, but the result of an immediate fiat of Deity. As far as regards the reverential thoughts due to the Divine Lawgiver of nature, it may appear, on a first view, unimportant whether we admit the creation of the complicated phenomena, visible in the structure of the globe, by an immediate act of Almighty Power, or their production from a former state of the same elements by the agency of intermediate laws of causation; but, on careful examination, it will certainly be found otherwise.
If it be true and demonstrated, that in the existing economy of nature all phenomena (whether they appear to our imperfect conceptions simple or complicated) are the result of invariable appointed laws, acting under definite conditions; if it appear that, in our own time, the phenomena of mechanical, chemical, and vital action among the elements and masses of matter are analogous to those of which monuments remain in the crust of the earth; if the laws which are known to govern and to correspond accurately to the modern effects stand in the same relation to those of older date; who, that looks upon the laws of modern nature, as affording proof of the being and attributes of God, will take a different view of the similar phenomena of ancient date, and thus virtually derogate from the respect due to the Lawgiver, by limiting the duration, and questioning the application of the law?
For it cannot be denied, that the appearances in the rocks which compose the crust of the globe, are such as to indicate most clearly that all their ingredients have existed in some other and earlier condition. The pebbles and fragments of stone imbedded in rocks of different nature are such as might be produced by previous mechanical action; crystals such as those imbedded in others are known to be effects of chemical forces; shells, plants, &c. retaining all their delicate external forms, and even their internal structure, can they be supposed to be mere lusus naturæ, or created to deceive mankind? Which is the more reasonable, to receive as truth the obvious indication of the senses, to acknowledge these effects to have happened through proximate causes, or to attribute to the Divine Wisdom the instantaneous creation of effects which, by their very nature and the nature of man, must inevitably mislead right reasoning to a wrong conclusion? It must, therefore, be allowed that the causes which the effects indicate, when rightly interpreted, are to be admitted as true; if the effects are rightly noted, and correctly interpreted, all the inferences of geology, however remarkable they may be, whatever agencies, conditions, or durations they assign to the composition of the crust of the earth, must be received as natural truths.
We may now follow the inquiry into the prior conditions of the materials consolidated in the crust of the earth. It is quickly seen that many considerable rocks are composed of parts which were suspended in water, as clays, sands, &c. and deposited from it as sediment; others are such as may be formed from solution in water; others resemble the products of igneous fusion; some appear the result of electrical combinations. All these latter are but forms of chemical processes among the elements of matter; and the sedimentary rocks, where their parts are clearly distinguishable, are found to be composed of grains or fragments, originally produced by aqueous, igneous, or electrical combination. Thus all the mineral masses of the earth known to us appear to have existed previously in a different state, when the elements were separated, so as to allow of their combination according to the forces of affinity, existing in definite proportions among the small portions of all material substance.
Take, for example, the very common rock sandstone; its component grains of quartz, felspar, and mica are, more or less, rolled or fragmented crystals of these substances, derived from rocks like gneiss, mica schist, &c., which are also composed of grains of the same kind, less affected by mechanical processes; or from granite, porphyry, &c., which are purely crystalline rocks. Such derivative sandstones are formed at this day from such crystallised granite, and other rocks. But the analysis goes further. Quartz is a compound of a metallic basis, silicium, and the air or gas oxygen. Felspar is a compound of silicium, calcium, potassium, &c., each united with its own proportion of oxygen. Mica is a compound of silicium, potassium, magnesium, calcium, &c. similarly combined with oxygen.
Under present physical conditions oxygen, being liberated from combination with these bases, would expand into 2000 times the bulk it occupies in the compound, and become a gas: and thus, since oxygen forms half the ponder able mass near the surface, half the crust of the globe, perhaps half its whole mass, would become an expansion or atmosphere round the diminished nucleus. It is evident that the tendency of all this inquiry is to lend some confirmation to the speculations of Herschel and Laplace, as to the condensation of the planetary masses from gaseous expansions, like the nebulæ and comets; speculations which, however, can only be changed into probable inferences by the progress of modern astronomy. For the examination of these obscure bodies, most powerful telescopes are required. The observations of Lord Rosse have resolved so many of the nebulæ into small stars, that it is doubtful whether any of them can be safely appealed to in illustration of the supposed process of condensation and arrangement.
The progress yet made in chemical philosophy is perhaps not such as to enable us to discover the single condition on which the elements, now so firmly united, could exist separately, in a free gaseous expansion; yet, since chemical combinations are known to be subject to temperature, liable to be altered and even reversed with a change of this condition, may we not suggest, as the least improbable view, that the nebulous condition of a planet may be due to intense heat existing among its particles; that, in fact, a great heat prevents their combination, and maintains them all together in a gaseous state, as it is known to be capable of doing, for most of them singly, and several of them together? In mixed or combined gases metallic matters are frequently present (as arseniuretted hydrogen), and the atmosphere of our planet is believed by several philosophers to contain so large a proportion of the substances existing in the superficial parts of the globe, as to give origin to the meteoric stones.