grit, afford a preservative against the ravages of accidental Fire beyond the area of that sheet in which it may take its beginning; but for such a provision, entire Coal fields might be occasionally burnt out and destroyed.

It is impossible to contemplate a disposition of things, so well adapted to afford the materials essential to supply the first wants, and to keep alive the industry of the Inhabitants of our earth; and entirely to attribute such a disposition to the blind operation of Fortuitous causes. Although indeed it be dangerous hastily to have recourse to Final causes, yet since in many branches of physical knowledge, (more especially in those which relate to organized matter,) the end of many a contrivance is better understood, than the contrivance itself, it would surely be as unphilosophical to hesitate at the admission of final Causes, when the general tenor and evidence of the Phenomena naturally suggest them, as it would be to introduce them gratuitously unsupported by such evidence. We may surely therefore feel ourselves authorized to view, in the Geological arrangements above described, a system of wise and benevolent Contrivances, prospectively subsidiary to the wants and comforts of the future inhabitants of the globe; and extending onwards, from its first Formation, through the subsequent Revolutions and Convulsions that have affected the surface of our Planet.

CHAPTER XXI.

Advantageous Effect of Disturbing Forces in giving Origin to Mineral Veins.*

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 1.-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 Ore. These fissures are more or less filled with various forms of metalliferous and earthy minerals,

See Pl. 1. Figs. k 1.-k 24, and Pl. 67. Fig. 3.

VEINS MOST FREQUENT in the older rocks. 549 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.*

* M. Dufrénoy has recently shewn that the mines of Hamatite and Spathic iron in the Eastern Pyrenees, which occur in Limestones of three ages, referrible 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 deposit 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 Fer de la Partie orientale des Pyrénées, 1834.)

According 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 fluid in the Tertiary period; and Tertiary strata which have been rendered crystalline by its heat, and are traversed by dykes 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.)

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 intermixt with other substances, and more advantageously wrought.*

Several hypotheses have been proposed to

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 Mr. De la Beche, under the appointment of the Board of Ordnance.

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.* A fourth hypothesis considers veins to have been slowly filled by Segregation, or infiltration; sometimes into contemporaneous cracks and cavities, formed

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 portion 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 be passed through heated Boracic Acid, it takes up and carries along with it a portion of the Acid, which per se does not sublime. This experiment illustrates the sublimation of Boracic Acid in volcanic craters.