Geology and Mineralogy considered with reference to Natural Theology/Chapter 21
CHAPTER XXI.
Advantageous Effect of Disturbing Forces in giving Origin to Mineral Veins.[1]
A further result attending the Disturbances of the surface of the Earth has been, to produce Rents or Fissures in the Rocks which have been subjected to these violent movements, and to convert them into receptacles of metallic ores, accessible by the labours of man. The greater part of metalliferous veins originated in enormous cracks and crevices, penetrating irregularly and obliquely downwards to an unknown depth, and resembling the rents and chasms which are produced by modern Earthquakes. The general disposition of mineral veins within these narrow fissures, will be best understood by reference to our first Section. (Pl. 1. Figs. k l.—k 24.) The narrow lines which pass obliquely from the lower to the upper portion of this Section, represent the manner in which Rocks of various ages are intersected by fissures, which have become the Receptacles of rich Treasures of Metallic Qre. These fissures are more or less filled with various forms of metalliferous and earthy minerals, deposited in successive, and often corresponding layers, on each side of the vein.
Metallic Veins are of most frequent occurrence in rocks
of the Primary. and Transition series, particularly in those
lower portions of stratified rocks which are nearest to unstratified
crystalline rocks. They are of rare occurrence
in Secondary formations, and still more so in Tertiary
strata.[2]
A few metals are occasionally, though rarely, found disseminated through the substance of Rocks. Thus Tin is sometimes found disseminated through Granite, and Copper through the cupriferous slate at the base of the Hartz, at Mansfeld, &c.
The most numerous and rich of the metallic veins in Cornwall, and in many other mining districts, are found near the junction of the Granite with the incumbent Slates. These vary in width from less than an inch to thirty feet and upwards; but the prevailing width, both of Tin and Copper Veins in that county, is from one to three feet; and in these narrower veins, the Ore is less intermixed with other substances, and more advantageously wrought.[3]
Several hypotheses have been proposed to explain the
manner in which these chasms in solid rocks have become
filled with metallic ores, and with earthy minerals, often of
a different nature from the rocks containing them. Werner
supposed that veins were supplied by matter descending
into them from above, in a state of aqueous solution; whilst
Hutton, and his followers, imagined that their contents were injected from below, in a state of igneous fusion. A third
hypothesis has been recently proposed, which refers the
filling of veins to a process of Sublimation from subjacent
masses of intensely heated mineral matter, into apertures
and fissures of the superincumbent Rocks.[4] A fourth hypothesis
considers veins to have been slowly filled by Segregation,
or infiltration; sometimes into contemporaneous
cracks and cavities, formed during the contraction and
consolidation of the originally soft substances of the rocks
themselves; and more frequently into fissures produced by
the fracture and dislocation of the solid strata. Segregation
of this kind may have taken place from electro-chemical
agency, continued during long periods of time.[5]
The total quantity of all metals known to exist near the surface of the Earth (excepting Iron,) being comparatively small, and their value to mankind being of the highest order, as the main instruments by the aid of which he emerges from the savage state, it was of the utmost importance, that they should be disposed in a manner that would render them accessible by his industry; and this object is admirably attained through the machinery of metallic veins.
Had large quantities of metals existed throughout Rocks of all formations, they might have been noxious to vegetation; had small quantities been disseminated through the Body of the Strata, they would never have repaid the cost of separation from the matrix. These inconveniences are obviated by the actual arrangement, under which these rare substances are occasionally collected together in the natural Magazines afforded by metallic veins.
In my Inaugural Lecture (page 12) I have spoken of the evidences of design and benevolent contrivance, .which are apparent in the original formation and disposition of the repositories of minerals; in the relative quantities in which they are distributed; in the provisions that are made to render them accessible, at a certain expense of human skill and industry, and at the same time secure from wanton destruction, and from natural decay; in the more general dispersion of those metals which are most important, and the comparatively rare occurrence of others which are less so; and still further. in affording the means whereby their compound ores may be reduced to a state of purity.[6]
The argument, however, which arises from the utility of these dispositions, does not depend on the establishment of any one or more of the explanations proposed to account for them. Whatever may have been the means whereby mineral veins were charged with their precious contents; whether Segregation, or Sublimation, were the exclusive method by which the metals were accumulated; or, whether each of the supposed causes may have operated simultaneously or consecutively in their production; the existence of these veins remains a fact of the highest importance to the human race: and although the Disturbances, and other processes in which they originated, may have taken place at periods long antecedent to the creation of our species, we may reasonably infer, that a provision for the comfort and convenience of the last, and most perfect creatures He was about to place upon its surface, was in the providential contemplation of the Creator, in his primary disposal of the physical forces, Which-have caused some of the.earliest, and most violent Perturbations of the globe.[7]
- ↑ See Pl. 1. Figs. k 1.—k 24, and Pl. 67. Fig. 3.
- ↑ M. Dufrenoy has recently shown that the mines of Hæmatite and Spathic iron in the Eastern Pyrenees, which occur in Limestones of three ages, referable severally to the Transition Series, to the Lias, and to the Chalk, are all situated in parts, where these Limestones are in near contact with the Granite; and he considers that they have all most probably been filled by the sublimation of mineral matter into cavities of the limestones, at, or soon after the time of the Elevation of the Granite of this part of the Pyrenees. The period of this elevation was posterior to the deposite of the Chalk formation, and anterior to that of the Tertiary Strata. These Limestones have all become crystalline where they are in contact with the Granite; and the Iron is in some places mixed with Copper pyrites, and argentiferous galena. (Mémoire sur la Position des Mines de Per de la Partie orientale des Pyrenees, 1834.)
Awarding to the recent observations of Mr. C. Darwin, the Granite of the Cordilleras of Chili (near the Uspellata Pass) which forms peaks of a height probably of 14,000 feet, has been Enid in the Tertiary period; and Tertiary strata which have been rendered crystalline by its heat, and are traversed by dikes from the granitic mass, are now inclined at high angles, and form regular, and complicated anticlinal lines. These same sedimentary strata, and also lavas are there traversed by very numerous true metallic veins of iron, copper, arsenic, silver, and gold, and these can be traced to the underlying granite. (Lond. and Edin. Phil. Mag. N. S. Vol. 8. p. 158.)
- ↑ An excellent illustration of the manner in which metallic veins are disposed in the Rocks which form their matrix, may be found in Mr. R. Thomas's geological Report, accompanied by a Map and Sections of the mining district near Redruth. This map comprehends the most interesting spot of all the mining districts in Cornwall, and exhibits in a small compass the most important phenomena of metallic veins, slides, and cross courses, all of them penetrating to an unknown depth, and continuing uninterruptedly through Rocks of various ages. In Pl. 67, Fig. 3, I have selected from this work a section, which exhibits an unusually dense accumulation of veins producing Tin, Copper, and Lead.
Much highly valuable information on these subjects may shortly be expected from the Geological Survey of Cornwall, now in progress by Hr. De la Beche, under the appointment of the Board of Ordnance.
- ↑ In the London and Edin. Phil. Mag. March, 1829, p. 172, Mr. Patterson has published the result of his experiments in making artificial Lead Ore (Galena) in an Earthen tube, highly heated in the middle. After causing the steam of water to pass over a quantity of Galena, placed in the hottest sport ion of this tube, the water was decomposed, and all the Galena had been sublimed from the heated part and deposited again in colder parts of the tube, in cubes which exactly resembled the original Ore. No pure Lead was formed. From this deposition of Galena, in a highly crystalline form, from its vapour in contact with steam, he draws the important conclusion, that Galena might, in some instances, have been supplied to mineral veins by sublimation from below.
Dr. Daubeny has found by a recent experiment that if steam he passed through heated Boracic Acid, it takes up and carries along with it a portion off the Acid, which per se does not sublime. This experiment illustrates the sublimation of Boracic Acid in volcanic craters.
- ↑
The observations of Mr. Fox on the electro-magnetic properties of metalliferous veins in Cornwall, (Phil. Trans. 1830, &c.) seem to throw new light upon this obscure and difficult subject. And the experiments of Ml Becquerel on the artificial production of crystallized insoluble compounds of Copper, Lead, Lime, &c. by the slow and long continued reaction and transportation of the elements of soluble compounds, (see Becquerel, Traité de l'Electricité, T. i. c. 7, page 547, 1834,) appear to explain many chemical changes that may have taken place under the influence of feeble electrical currents in the interior of the earth, and more especially in Veins.
I have been favoured by Professor Wheatstone with the following brief explanation of the experiments here quoted.
"When two bodies, one of which is liquid, react very feebly on each other, the presence of a third body, which is either a conductor of electricity, or in which capillary action supplies the place of conductibility opens a path to the electricity resulting from the chemical action, and a voltaic current is formed which serves to augment the energy of the chemical action of the two bodies. In ordinary chemical actions, combinations are affected by the direct reaction of bodies on each other, by which all their constituents simultaneously concur to the general effect; but in the mode considered by Becquerel the bodies in the nascent state, and excessively feeble forces, are employed by which the molecules are produced, as it were, one by one, and are disposed to assume regular forms, even when they are insoluble, because the number of the molecules cannot occasion any disturbance in their arrangement. By the application of these principles, that is, by the long-continued action of very feeble electrical currents, this author has shown that many crystallized bodies, hitherto found only in nature, may be artificially obtained."
- ↑ I owe to my friend Mr. John Taylor the suggestion of another argument, arising from the phenomena of mines, which derives much value from being the result of t.he long experience of a practical man of science. "There is one argument," says Mr. Taylor, "which has always struck me with considerable force, as proving wise and beneficent design, to be drawn from the position of the metals. I should say that they are so placed as to be out of the reach of immediate and improvident exhaustion, exercising the utmost ingenuity of man, first to discover them, then to devise means of conquering the difficulties by which the pursuit of them is surrounded. "Hence a continued supply through successive ages, and hence motives to industry and to the exercise of mental faculties; from which our greatest happiness is derived. The metals might have been so placed as to have been all easily taken away, causing a glut in some periods and a dearth in others, and they might have been accessible without thought, or ingenuity. "As they are, there appears to be that accordance with the perfect arrangement of an all-wise Creator, which it is so beautiful to observe and to contemplate."
- ↑ That part of the History of Metals which relates to their various Properties and Uses, and their especial Adaptation to the Physical condition of Man, has been so ably and amply illustrated by two of my Associates in this Series of Treatises, that I have more satisfaction in referring my readers to the Chapters of Dr. Kidd and Dr. Prout upon these subjects than in attempting myself to follow the history of the productions of metallic veins, beyond the sources from which they are derived within the body of the Earth.
A summary of the all-important Uses of Metals to Mankind is thus briefly given, by one of our earliest and most original writers on Physical-theology.
"As for Metals, they are so many ways useful to mankind, and those Uses so well known to all, that it would be lost labour to say anything of them: without the use of these we could have nothing of culture or civility: no Tillage or Agriculture; no Reaping or Moving; no Ploughing or Digging; no Pruning or Loping; no Gratting or Insition; no mechanical Arts or Trades; no Vessels or Utensils of Household-stuff; no convenient Houses or Edifices; no Shipping or Navigation. What a kind of barbarous and sordid life we must necessarily have lived, the Indians in the Northern part of America are a clear demonstration. Only it is remarkable that those which are of most frequent and necessary use, as Iron, Brass and Lead, are the most common and plentiful: others that are more rare, may better be spared, yet are they thereby qualified to be made the common measure and standard of the value of all other commodities, and so to serve for Coin or Money, to which use they have been employed by all civil Nations in all Ages." Ray's Wisdom of God in the Creation. Pt. i. 5th ed; 1709, p. 110.