SECTION VI.

GEOLOGICAL PHENOMENA RESULTING FROM THE EARTH'S

PROTOROTATION.

CHAPTER XXII.

FAULTS OF FISSURES described. Geological evidences of their existence. Application of these data to the COAL MEASURES, considered to have been the uppermost strata of the Non-rotatory Sphere. Found to correspond. METALLIC VEINS, described; geological and other scientific data descriptive of these interesting portions of the rocky crust of the earth.

HAVING thus been enabled to indicate the probable origin of the mineral veins which proceed in directions from the centre towards the circumference, we shall in this chapter endeavour to explain, as for as the Dynamical Theory will enable us, whence we consider those called "fissures" or "faults," which proceed in a contrary direction, or from the surface towards the interior of the earth's crust, to have arisen. They are taken notice of in the thirtieth Theorem, and its accompanying evidence, which state, "That two distinct classes of mineral veins are found to exist in the earth's outer crust-one of which proceeds from inwards outwards, having their bases in the interior, and their apici nearest to the surface; and the other termed Faults and Fissures proceeding from outwards inwards, with their apici in the interior and their bases on or near to the surface."

"In our last chapter," says Professor Buckland, "we considered the advantages of the disposition of the carboniferous strata in the form of basins. It remains to examine the further advantages that arise from other disturbances of these strata by faults or fractures, which are of great importance in facilitating the operations of coal

mines. The component strata of a coal field are divided into insulated masses, or sheets of rock, of irregular form and area, not one of which is continuous in the same plane over any very large district; but each is usually separated from its next adjacent mass by a dam of clay, impenetrable to water, and filling the fissure produced by the fracture which caused the fault.

"If we suppose a thick sheet of ice to be broken into fragments of irregular areas, and these fragments again united, after receiving a slight degree of irregular inclination to the plane of the original sheet, the re-united fragments of ice will represent the appearance of the component portions of the broken masses, or sheets of coal measures we are describing. The intervening portions of more recent ice, by which they are held together, represent the clay and rubbish that fill the faults, and form the partition walls that insulate these adjacent portions of strata which were originally formed, like the sheet of ice, in one continuous plane. Thus, each sheet, or inclined table of coal measures, is inclosed by a system of more or less vertical walls of broken clay, derived from its argillaceous shale beds, at the moment at which the fracture and dislocation took place; and hence have resulted those joints and separations which, though they occasionally interrupt at inconvenient positions, and cut off suddenly the progress of the collier, and often shatter those portions of the strata that are in immediate contact with them, yet are, in the main, his greatest safeguard, and are, indeed, essential to his operations."*

"The immense violence," observes Professor Playfair," which has accompanied the formation of mineral veins, is particularly marked by the slips, and shifts of the strata on each side of them, all tending to show that mighty changes have taken place in those regions, which our imagination erroneously paints as the abode of silence and rest.

Mineral veins contain abundant marks of the most violent and repeated disturbance. And it appears most likely, that fissures in the strata were made, at least in many instances, and the matter poured into them, nearly at the same time, both being effects of the same cause, the expansive force of subterraneous heat. . .

"If all these circumstances are put together, there appears but one conclusion to be drawn from them. The manifest marks of some power which could lift up fragments of rocks from their native

* Bridgewater Treatise, vol. i. pp. 541-544.

places, distant several hundred yards from their present situations; place them upright on their edges, encompass them with solid rock quite heterogeneous to themselves, and bestow upon them a great addition of solidity and induration.

"It is indeed impossible, that the effects of motion and heat can be more clearly expressed than by these symptoms; or the subject in which these powers resided, more distinctly pointed out."*

"Faults," according to Mr. Conybeare, "consist of fissures, traversing the strata, extending often for several miles, and penetrating to a depth in very few instances ascertained; they are accompanied by a subsidence of the strata on one side of their line, or (which amounts to the same thing) an elevation of them on the other; so that it appears, that the same force which has rent the rocks thus asunder, has caused one side of the fractured mass to rise, or the other to sink. The fissures are generally filled by clay."+

"As we can scarcely conceive such a general and simultaneous movement of the inferior strata," says M. de la Beche, when treating of the red sandstone group, "immediately preceding the first deposits of the red sandstone series, that every point on which it reposes was convulsed and threw off fragments on the sudden elevation of lines of strata, we should rather look to certain foci of disturbance for the dispersion of debris. The accumulation of the larger fragments, and the relative amount of conglomerate, would, under this hypothesis, be greatest nearest to the disturbing cause; and amid such turmoil we might anticipate the occurrence of igneous rocks thrown up at the same period. . . . But notwithstanding the abundance of the greenstones and dark coloured porphyries, not a fragment of them has yet been discovered among the conglomerates, though rolled portions of the red porphyries are so abundant. This fact seems to attest that the dark coloured trappean rocks did not exist in such a state, when fragments of slate, limestone, &c. &c., were broken off, that they could be fractured and broken with the rest: though it does not show that trap rocks may not have been protruded at the time of convulsion, thus aiding the confusion, and in a great measure causing it. On the contrary, we have every reason to consider that the eruption of trap rocks did accompany (if partly not produce), the disruption of strata, whence the fragments in the conglomerate were derived: for

* Illustrations of Hutt. Theory, Playfair's Works, vol. i. pp. 76, 258, 301. + Geology of England and Wales, part i. p. 348.

we have seen that red quartziferous porphyry, in mass, surmounts a portion of the red conglomerate; and the occurrence of trappean rocks so blended with the conglomerates that lines of separation cannot be drawn between them. Now, if igneous rocks were ejected, a conclusion which the facts appear to justify, at the time of the production of the conglomerate, there would seem no reason why, under favourable circumstances, the two should not be in some measure blended with each other. Another circumstance also lends probability to this view, and that is, the occurrence of pebbles cemented in certain inferior beds by a kind of semi-trappean paste, containing crystals of that variety of felspar named murchisonite by Mr. Levi. Such a cement might possibly have resulted from the upburst of igneous rocks, accompanied by various gases beneath a mass of water, when some of the erupted matter may have so combined as to form a cement, in which crystals of murchisonite became developed without some such hypothesis this cement seems of very difficult explanation.”*

"Numerous rents may often be seen," says Mr. Lyell, "in rocks which appear to have been simply broken, the separated parts remaining in the same places; but we often find a fissure, several inches or yards wide, intervening between the disunited portions.

"These fissures are usually filled with fine earth and sand, or with angular fragments of stone, evidently derived from the fracture of the contiguous rocks.

"It is not uncommon to find the mass of rock on one side of a fissure thrown up above, or down below, the mass with which it was once in contact on the other side. This mode of displacement is called a shift, slip, or fault.

"The miner,' says Playfair, describing a fault, 'is often perplexed in his subterraneous journey, by a derangement in the strata, which changes at once all those lines and bearings which had hitherto directed his course.'

"We may occasionally see exact counterparts of these slips, on a small scale, in pits of fine loose sand and gravel, many of which have doubtless been caused by the drying and shrinking of argillaceous and other beds, slight subsidences having taken place from failure of support.

"In the present state of our ignorance of the causes of subsidence, an hypothesis which can explain the great amount of displacement

* Manual of Geology, pp. 403, 404.

in some faults, on sound mechanical principles, by a succession of movements, is far preferable to any theory which assumes each fault to have been accomplished by a single upcast or downthrow of several thousand feet. For, we know that there are operations now in progress at great depths in the interior of the earth, by which both large and small tracts of ground are made to rise above and sink below their former level, some slowly and insensibly, others suddenly and by starts, a few feet or yards at a time; whereas there are no grounds for believing that, during the last 3,000 years at least, any regions have been either upheaved or depressed, at a single stroke, to the amount of several hundred, much less several thousand feet."*

"In some cases," says Professor Phillips, "instead of acclinal or declinal slopes to or from an axis, we have a complete fracture of the mass of strata along a vertical or inclined plane, parallel to which the beds on one side are uplifted, and on the other depressed. This is called a fault or slip; almost every coal district and mining region in the world is full of such, though their number is, upon the whole, very much greatest in elevated districts, and least in the youngest strata.

"The extent of displacement on one side of such fault is sometimes only a few inches; in other cases 10, 100, or 1,000 feet or yards. The great Craven fault and Cross Fell fault in the North of England is complicated with a narrow anticlinal axis, the extent of displacement produced by both is 1,000, 2,000, 3,000, or even 4,000 feet (Diagram, No. 7).

"The jointed structure of rocks of the carboniferous system has been minutely investigated. In the Geology of Yorkshire, vol. ii., it is shown, from eighty-five observations in the carboniferous system, that in the mountain limestone and coal tracts of Yorkshire, the long joints affect certain principal directions, so that two positive axes, in which these divisional planes are most frequent, are traced at right angles to one another; and two negative axes in which no long joints have been observed, also at right angles to each other.

"The axes of frequent joints run N.N.W. and S.S.E., and E.N.E. and W.S.W., the negative axes are N.E. by N., and N.W. by W. This singular result of observation harmonizes with the principal directions of mineral veins in the district bordering on the great Cross Fell and Craven faults; it also bears a close analogy with the

Elements, vol. i. pp. 128, 129, 131, 135.