situations where it would be detrimental, are at the same time of the greatest service, in converting it to purposes of utility, by creating on the surface a series of Springs along the line of Fault, which often give notice of the Fracture that has taken place beneath. This important effect of Faults on the hydraulic machinery of the globe extends through stratified rocks of every formation. (See Pl. 69. Fig. 2.) It is also probable that most of the Springs, that issue from unstratified rocks, are kept in action through the instrumentality of the Faults by which they are intersected.

A similar interruption of continuity in the masses of Primary rocks, and in the rocks of intermediate age between these and the Coal formation, is found to occur extensively in the working of metallic veins. A vein is often cut off suddenly by a Fault, or fracture, crossing it trans

castle, on the wet side of the 90 fathom Dyke, and was so inundated with water that it was soon found necessary to abandon it. Another shaft was then begun on the dry side of the dyke, only a few yards from the former, and in this they descended nearly 200 fathoms without any impediment from water.

Artificial dams are sometimes made in coal mines to perform the office of the natural barriers which Dykes and Faults supply. A dam of this kind was lately made near Manchester, by Mr. Hulton, to cut off water that descended from the upper regions of porous strata, which dipped towards his excavations in a lower region of the same strata, the continuity of which was thus artificially interrupted.

versely, and its once continuous portions are thrown to a considerable distance from each other. This line of fracture is usually marked by a wall of clay, formed probably by the abrasion of the rocks whose adjacent portions have been thus dislocated. Such faults are known in the mines of Cornwall by the term flucan, and they often produce a similar advantage to those that traverse the Coal measures, in guarding the miner from inundation, by a series of natural dams traversing the rocks in various directions, and intercepting all communication between that mass in which he is conducting his operations, and the adjacent masses on the other side of the flucan or dam.*

It may be added also, that the Faults in a Coal field, by interrupting the continuity of the beds of coal, and causing their truncated edges to abut against those of uninflammable strata of shale or

"My object is rather to suggest whether the arrangement of veins, &c. does not argue design and a probable connection with other phenomena of our Globe.

“Metalliferous veins, and those of quartz, &c. appear to be channels for the circulation of the subterraneous water and vapour; and the innumerable clay veins, or "flucan courses" (as they are termed in Cornwall), which intersect them, and are often found contained in them, being generally impervious to water, prevent their draining the surface of the higher grounds as they otherwise would, and also facilitate the working of mines to a much greater depth than would be practicable without them." R. W. Fox on the Mines of Cornwall, Phil. Trans. 1830, p. 404.

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.

Al

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. though 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.)