A Manual of Elementary Geology Part 75

The same observation applies to killas and the granitic porphyry called elvan. Sometimes, in the same continuous vein, the granite will contain copper, and the killas tin, or _vice versa_."[497-A] Mr. Fox, after ascertaining the existence at present of electric currents in some of the metalliferous veins in Cornwall, has speculated on the probability of the same cause having acted originally on the sulphurets and muriates of copper, tin, iron, and zinc, dissolved in the hot water of fissures, so as to determine the peculiar mode of their distribution. After inst.i.tuting experiments on this subject, he even endeavoured to account for the prevalence of an east and west direction in the princ.i.p.al Cornish lodes by their position at right angles to the earth's magnetism; but Mr. Henwood and other experienced miners have pointed out objections to the theory; and it must be owned that the direction of veins in different mining districts varies so entirely that it seems to depend on lines of fracture, rather than on the laws of voltaic electricity. Nevertheless, as different kinds of rock would be often in different electrical conditions, we may readily believe that electricity must often govern the arrangement of metallic precipitates in a rent.

"I have observed," says Mr. R. Fox, "that when the chloride of tin in solution is placed in the voltaic circuit, part of the tin is deposited in a metallic state at the negative pole, and part at the positive one, in the state of a peroxide, such as it occurs in our Cornish mines. This experiment may serve to explain why tin is found contiguous to, and intermixed with, copper ore, and likewise separated from it, in other parts of the same lode."[497-B]

_Relative age of the different metals._--After duly reflecting on the facts above described, we cannot doubt that mineral veins, like eruptions of granite or trap, are referable to many distinct periods of the earth's history, although it may be more difficult to determine the precise age of veins; because they have often remained open for ages, and because, as we have seen, the same fissure, after having been once filled, has frequently been re-opened or enlarged. But besides this diversity of age, it has been supposed by some geologists that certain metals have been produced exclusively in earlier, others in more modern times,--that tin, for example, is of higher antiquity than copper, copper than lead or silver, and all of them more ancient than gold. I shall first point out that the facts once relied upon in support of some of these views are contradicted by later experience, and then consider how far any chronological order of arrangement can be recognized in the position of the precious and other metals in the earth's crust. In the first place, it is not true that veins in which tin abounds are the oldest lodes worked in Great Britain. The government survey of Ireland has demonstrated, that in Wexford veins of copper and lead (the latter as usual being argentiferous) are much older than the tin of Cornwall. In each of the two countries a very similar series of geological changes has occurred at two distinct epochs,--in Wexford, before the Devonian strata were deposited; in Cornwall, after the carboniferous epoch. To begin with the Irish mining district: We have granite in Wexford, traversed by granite veins, which veins also intrude themselves into the Silurian strata, the same Silurian rocks as well as the veins having been denuded before the Devonian beds were superimposed. Next we find, in the same county, that elvans, or straight dikes of porphyritic granite, have cut through the granite and the veins before mentioned, but have not penetrated the Devonian rocks. Subsequently to these elvans, veins of copper and lead were produced, being of a date certainly posterior to the Silurian, and anterior to the Devonian; for they do not enter the latter, and, what is still more decisive, streaks or layers of derivative copper have been found near Wexford in the Devonian, not far from points where mines of copper are worked in the Silurian strata.[498-A]

Although the precise age of such copper lodes cannot be defined, we may safely affirm that they were either filled at the close of the Silurian or commencement of the Devonian period. Besides copper, lead, and silver, there is some gold in these ancient or primary metalliferous veins. A few fragments also of tin found in Wicklow in the drift are supposed to have been derived from veins of the same age.[498-B]

Next, if we turn to Cornwall, we find there also the monuments of a very a.n.a.logous sequence of events. First the granite was formed; then, about the same period, veins of fine-grained granite, often tortuous (see fig.

496., p. 445.), penetrating both the outer crust of granite and the adjoining fossiliferous or primary rocks, including the coal-measures; thirdly, elvans, holding their course straight through granite, granitic veins, and fossiliferous slates; fourthly, veins of tin also containing copper, the first of those eight systems of fissures of different ages already alluded to, p. 491. Here, then, the tin lodes are newer than the elvans. It has indeed been stated by some Cornish miners that the elvans are in some few instances posterior to the oldest tin-bearing lodes, but the observations of Sir H. De la Beche during the survey led him to an opposite conclusion, and he has shown how the cases referred to in corroboration can be otherwise interpreted.[499-A] We may, therefore, a.s.sert that the most ancient Cornish lodes are younger than the coal-measures of that part of England, and it follows that they are of a much later date than the Irish copper and lead of Wexford and some adjoining counties. How much later it is not so easy to declare, although probably they are not newer than the beginning of the Permian period, as no tin lodes have been discovered in any red sandstone of the Poikilitic group, which overlies the coal in the south-west of England.

There are lead veins in the Mendip hills which extend through the mountain limestone into the Permian or Dolomitic conglomerate, and others in Glamorganshire which enter the lias. Those worked near Frome, in Somersetshire, have been traced into the Inferior Oolite. In Bohemia, the rich veins of silver of Joachimsthal cut through basalt containing olivine, which overlies tertiary lignite, in which are leaves of dicotyledonous trees. This silver, therefore, is decidedly a tertiary formation. In regard to the age of the gold of the Ural Mountains, in Russia, which, like that of California, is obtained chiefly from auriferous alluvium, we can merely affirm that it occurs in veins of quartz in the schistose and granitic rocks of that chain. Sir R. Murchison observes, that no gold has yet been found in the Permian conglomerates which lie at the base of the Ural Mountains, although large quant.i.ties of iron and copper detritus are mixed with the rolled pebbles of these same Permian strata. Hence it seems that the Uralian quartz veins, containing gold and platinum, were not exposed to aqueous denudation during the Permian era. But we cannot feel sure, from any data yet before us, that such auriferous veins of quartz may not be as old as the tin lodes of Cornwall, in which, as well as the more ancient copper lodes of Ireland, some gold has been detected. We are also unable at present to a.s.sign to the gold veins of Brazil, Peru, or California, their respective geological dates. But, although enough is known to show that Ovid's line about the "Age of Gold," "Aurea prima sata est aetas," would, by no means, be an apt motto for a treatise on mining, it would be equally rash in the present state of our inquiries to affirm, as some have done, that gold was the last-formed of metals.

It has been remarked by M. de Beaumont, that lead and some other metals are found in dikes of basalt and greenstone, as well as in mineral veins connected with trap rocks, whereas tin is met with in granite and in veins a.s.sociated with the granitic series. If this rule hold true generally, the geological position of tin in localities accessible to the miners will belong, for the most part, to rocks older than those bearing lead. The tin veins will be of higher relative antiquity for the same reason that the "underlying" igneous formations or granites which are visible to man are older, on the whole, than the overlying or trappean formations.

If different sets of fissures, originating simultaneously at different levels in the earth's crust, and communicating, some of them, with volcanic, others with heated plutonic ma.s.ses, be filled with different metals, it will follow that those formed farthest from the surface will usually require the longest time before they can be exposed superficially.

In order to bring them into view, or within reach of the miner, a greater amount of upheaval and denudation must take place in proportion as they have lain deeper when first formed. A considerable series of geological revolutions must intervene before any part of the fissure, which has been for ages in the proximity of the plutonic rocks, so as to receive the gases discharged from it when it was cooling, can emerge into the atmosphere. But I need not enlarge on this subject, as the reader will remember what was said in the 30th, 34th, and 37th chapters, on the chronology of the volcanic and hypogene formations.

_Concluding Remarks._--The theory of the origin of the hypogene rocks, at a variety of successive periods, as expounded in two of the chapters just cited, and still more the doctrine that such rocks may be now in the daily course of formation, has made and still makes its way, but slowly, into favour. The disinclination to embrace it has arisen partly from an inherent obscurity in the very nature of the evidence of plutonic action when developed on a great scale, at particular periods. It has also sprung, in some degree, from extrinsic considerations; many geologists having been unwilling to believe the doctrine of the trans.m.u.tation of fossiliferous into crystalline rocks, because they were desirous of finding proofs of a beginning, and of tracing back the history of our terraqueous system to times anterior to the creation of organic beings. But if these expectations have been disappointed, if we have found it impossible to a.s.sign a limit to that time throughout which it has pleased an Omnipotent and Eternal Being to manifest his creative power, we have at least succeeded beyond all hope in carrying back our researches to times antecedent to the existence of man. We can prove that man had a beginning, and that, all the species now contemporary with man, and many others which preceded, had also a beginning, and that, consequently, the present state of the organic world has not gone on from all eternity, as some philosophers have maintained.

It can be shown that the earth's surface has been remodelled again and again; mountain chains have been raised or sunk; valleys formed, filled up, and then re-excavated; sea and land have changed places; yet throughout all these revolutions, and the consequent alterations of local and general climate, animal and vegetable life has been sustained.

This has been accomplished without violation of the laws now governing the organic creation, by which limits are a.s.signed to the variability of species. The succession of living beings appears to have been continued not by the trans.m.u.tation of species, but by the introduction into the earth from time to time of new plants and animals, and each a.s.semblage of new species must have been admirably fitted for the new states of the globe as they arose, or they would not have increased and multiplied and endured for indefinite periods.[501-A]

Astronomy had been unable to establish the plurality of habitable worlds throughout s.p.a.ce, however favourite a subject of conjecture and speculation; but geology, although it cannot prove that other planets are peopled with appropriate races of living beings, has demonstrated the truth of conclusions scarcely less wonderful,--the existence on our own planet of so many habitable surfaces, or worlds as they have been called, each distinct in time, and peopled with its peculiar races of aquatic and terrestrial beings.

The proofs now acc.u.mulated of the close a.n.a.logy between extinct and recent species are such as to leave no doubt on the mind that the same harmony of parts and beauty of contrivance which we admire in the living creation, has equally characterized the organic world at remote periods. Thus as we increase our knowledge of the inexhaustible variety displayed in living nature, and admire the infinite wisdom and power which it displays, our admiration is multiplied by the reflection, that it is only the last of a great series of pre-existing creations, of which we cannot estimate the number or limit in times past.[501-B]

FOOTNOTES:

[489-A] Principles, &c. chap. iv. 8th ed. p. 49.

[491-A] Geol. Trans. vol. iv. p. 139.; Trans. Roy. Geol. Society Cornwall, vol. ii. p. 90.

[492-A] Carne, Trans. of Geol. Soc. Cornwall, vol. iii. p. 238.

[492-B] Fournet, Etudes sur les Depots Metalliferes.

[493-A] Geol. Rep. on Cornwall, p. 340.

[493-B] Principles, ch. xxvii. 8th ed. p. 422.

[496-A] See Dr. Daubeny's Volcanos.

[496-B] Bulletin, iv. p. 1278.

[497-A] R. W. Fox on Mineral Veins, p. 10.

[497-B] Ibid. p. 38.

[498-A] I am indebted to Sir H. De la Beche for this information. See also maps and sections of Irish Survey.

[498-B] Sir H. De la Beche, MS. notes on Irish Survey.

[499-A] Report on Geology of Cornwall, p. 310.

[501-A] See Principles of Geol., Book 3.

[501-B] See the author's Anniv. Address to the Geol. Soc. 1837. Proceedings of G. S. No. 49. p. 520.