Charles Lyell

“ 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 fault, shift, slip, or throw." "The miner," says Playfair, describing a fault, "is often perplexed, in his subterranean journey, by a derangement in the strata, which changes at once all those lines and bearings which had hitherto directed his course. When his mine reaches a certain plane, which is sometimes perpendicular, as in A B, Figure 74, sometimes oblique to the horizon (as in C D, ibid.) ”

“ The face of each wall of the fissure is often beautifully polished, as if glazed, striated, or scored with parallel furrows and ridges, such as would be produced by the continued rubbing together of surfaces of unequal hardness. ”

“ The emergence of strata at the surface is called by miners their OUTCROP, or BASSET.
If, instead of being folded into parallel ridges, the beds form a boss or dome-shaped protuberance, and if we suppose the summit of the dome carried off, the ground-plan would exhibit the edges of the strata forming a succession of circles, or ellipses, round a common centre. These circles are the lines of strike, and the dip being always at right angles is inclined in the course of the circuit to every point of the compass, constituting what is termed a qua-quaversal dip-- that is, turning every way. ”

“ An hypothesis which attributes such a change of position to 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 ”

“ If not provided with a clinometer, a most useful instrument, when it is of consequence to determine with precision the inclination of the strata, the observer may measure the angle within a few degrees by standing exactly opposite to a cliff where the true dip is exhibited, holding the hands immediately before the eyes, and placing the fingers of one in a perpendicular, and of the other in a horizontal position, as in Figure 62. ”

“ But the edges of highly inclined strata may give rise to perfectly horizontal lines in the face of a vertical cliff, if the observer see the strata in the line of the strike, the dip being inward from the face of the cliff. If, however, we come to a break in the cliff, which exhibits a section exactly at right angles to the line of the strike, we are then able to ascertain the true dip. ”

“ On the removal of the weight, the layers of clay were found to be curved and folded, so as to bear a miniature resemblance to the strata in the cliffs. We must, however, bear in mind that in the natural section or sea-cliff we only see the foldings imperfectly, one part being invisible beneath the sea, and the other, or upper portion, being supposed to have been carried away by DENUDATION, or that action of water which will be explained in the next chapter. ”

“ It is also found that below the creeps a, b, c, d, an inferior stratum, called the "metal coal," which is 3 feet thick, has been fractured at the points e, f, g, h, and has risen, so as to prove that the upward movement, caused by the working out of the "main coal," has been propagated through a thickness of 54 feet of argillaceous beds, which intervene between the two coal-seams. This same displacement has also been traced downward more than 150 feet below the metal coal, but it grows continually less and less until it becomes imperceptible. ”

“ Through the centre of this valley runs an imaginary line A, called technically a "synclinal line," where the beds, which are tilted in opposite directions, may be supposed to meet. It is most important for the observer to mark such lines, for he will perceive by the diagram that, in travelling from the north to the centre of the basin, he is always passing from older to newer beds; whereas, after crossing the line A, and pursuing his course in the same southerly direction, he is continually leaving the newer, and advancing upon older strata. ”

“ Or if we bore vertically through the chalk of the North Downs, we come, after traversing marine chalky strata, upon a fresh-water formation many hundreds of feet thick, called the Wealden, such as is seen in Kent and Surrey, which is known in its turn to rest on purely marine beds. In like manner, in various parts of Great Britain we sink vertical shafts through marine deposits of great thickness, and come upon coal which was formed by the growth of plants on an ancient land-surface sometimes hundreds of square miles in extent. ”

“ Now it might have been expected that, instead of the floor rising up, the ceiling would sink down, and this effect, called a "thrust," does, in fact, take place where the pavement is more solid than the roof. But it usually happens, in coalmines, that the roof is composed of hard shale, or occasionally of sandstone, more unyielding than the foundation, which often consists of clay. ”

“ Oftentimes a great line of fault has been repeated, or the movements have been continued through successive periods, so that, newer deposits having covered the old line of displacement, the strata both newer and older have given way along the old line of fracture. ”

“ Darwin has also inferred that, in those seas where circular coral islands and barrier reefs abound, there is a slow and continued sinking of the submarine mountains on which the masses of coral are based; while there are other areas of the South Sea where the land is on the rise, and where coral has been upheaved far above the sea-level. ”

“ It is certainly not easy to understand how in the subterranean regions one mass of solid rock should have been folded up by a continued series of movements, while another mass in contact, or only separated by a line of fissure, has remained stationary or has perhaps subsided. ”

“ The "creeps," as they are called in coal-mines, afford an excellent illustration of this fact.-- First, it may be stated generally, that the excavation of coal at a considerable depth causes the mass of overlying strata to sink down bodily, even when props are left to support the roof of the mine. "In Yorkshire," says Mr. Buddle, "three distinct subsidences were perceptible at the surface, after the clearing out of three seams of coal below, and innumerable vertical cracks were caused in the incumbent mass of sandstone and shale which thus settled down." ”

“ I have already stated that a geologist must be on his guard, in a region of disturbed strata, against inferring repeated alternations of rocks, when, in fact, the same strata, once continuous, have been bent round so as to recur in the same section, and with the same dip. ”

“ We have already explained, Figure 69, that stratified rocks have usually their strata bent into parallel folds forming anticlinal and synclinal axes, a group of several of these folds having often been subjected to a common movement, and having acquired a uniform strike or direction. In some disturbed regions these folds have been doubled back upon themselves in such a manner that it is often difficult for an experienced geologist to determine correctly the relative age of the beds by superposition. ”

“ We also know, especially by the study of regions liable to earthquakes, that there are causes at work in the interior of the earth capable of producing a sinking in of the ground, sometimes very local, but often extending over a wide area. The continuance of such a downward movement, especially if partial and confined to linear areas, may produce regular folds in the strata. ”

“ No European chain of mountains affords so remarkable an illustration of the persistency of such flexures for a great distance as the Appalachians before alluded to, and none has been studied and described by many good observers with more accuracy. ”

“ The usual absence of protruding masses of rock forming precipices or ridges along the lines of great faults has already been alluded to in explaining Figure 76, and the same remarkable fact is well exemplified in every coal-field which has been extensively worked. ”

“ Buddle, is a slight curvature at the bottom of each gallery, as at a, Figure 59: then the pavement, continuing to rise, begins to open with a longitudinal crack, as at b; then the points of the fractured ridge reach the roof, as at c; and, lastly, the upraised beds close up the whole gallery, and the broken portions of the ridge are reunited and flattened at the top, exhibiting the flexure seen at d. Meanwhile the coal in the props has become crushed and cracked by pressure. ”

“ The upper part of the curves seen in this diagram, Figure 73, and expressed in fainter lines, has been removed by what is called denudation, to be afterwards explained.
FRACTURES OF THE STRATA AND FAULTS.
Numerous rents may often be seen in rocks which appear to have been simply broken, the fractured parts still remaining in contact; 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. ”

“ The strike of the parallel ridges, after continuing in a straight line for many hundred miles, is then found to vary for a more limited distance as much as 30 degrees, the folds wheeling round together in the new direction and continuing to be parallel, as if they had all obeyed the same movement. ”

“ It would be natural to expect the fracture of solid rocks to take place chiefly where the bending of the strata has been sharpest, and such rending may produce ravines giving access to running water and exposing the surface to atmospheric waste. ”

“ The manner in which the numerous parallel folds, all simultaneously formed, assume a new direction common to the whole of them, and sometimes varying at an angle of 30 degrees from the normal strike of the chain, shows what deviation from an otherwise uniform strike of the beds may be experienced when the geographical area through which they are traced is on so vast a scale. ”

“ Our inability to explain the nature of the process is, perhaps, not simply owing to the inaccessibility of the subterranean regions where the mechanical force is exerted, but to the extreme slowness of the movement. ”

“ In this case the newer strata may rest for the most part conformably on the older, but, extending farther, pass over their edges. Every intermediate state between unconformable and over-lapping beds may occur, because there may be every gradation between a slight derangement of position, and a considerable disturbance and denudation of the older formation before the newer beds come on. ”

“ Vertical strata, when they can be traced continuously upward or downward for some depth, are almost invariably seen to be parts of great curves, which may have a diameter of a few yards, or of several miles. ”

“ It has been shown in the third chapter that there is such a difference between land, fresh-water, and marine fossils as to enable the geologist to determine whether particular groups of strata were formed at the bottom of the ocean or in estuaries, rivers, or lakes. ”

“ First, it will account equally for the position of those elevated masses of marine origin in which the stratification remains horizontal, and for those in which the strata are disturbed, broken, inclined, or vertical. Secondly, it is consistent with human experience that land should rise gradually in some places and be depressed in others. ”

“ If such be the case, we have scarcely more reason to expect to witness the operation of the process within the limited periods of our scientific observation than to see the swelling of the roots of a tree, by which, in the course of years, a wall of solid masonry may be lifted up, rent or thrown down. ”

“ And while these proofs of continental elevation and subsidence, by slow and insensible movements, have been recently brought to light, the evidence has been daily strengthened of continued changes of level effected by violent convulsions in countries where earthquakes are frequent. ”

“ Section of carboniferous strata at Wallsend, Newcastle, showing "creeps." (J. Buddle, Esq.) Horizontal length of section 174 feet. The upper seam, or main coal, here worked out, was 630 feet below the surface. Section through, from top to bottom: Siliceous sandstone. Shale. 1. Main coal, 6 feet 6 inches, with creeps a, b, c, d. Shale eighteen yards thick. 2. Metal coal, 3 feet, with fractures e, f, g, h.)
When a bed of coal is worked out, pillars or rectangular masses of coal are left at intervals as props to support the roof, and protect the colliers. ”

“ There the rocks are rent from time to time, and heaved up or thrown down several feet at once, and disturbed in such a manner as to show how entirely the original position of strata may be modified in the course of centuries. ”

“ Strata of Deep-sea and Shallow-water Origin alternate. Also Marine and Fresh-water Beds and old Land Surfaces. Vertical, inclined, and folded Strata. Anticlinal and Synclinal Curves. Theories to explain Lateral Movements. Creeps in Coal-mines. Dip and Strike. Structure of the Jura. Various Forms of Outcrop. Synclinal Strata forming Ridges. Connection of Fracture and Flexure of Rocks. Inverted Strata. Faults described. Superficial Signs of the same obliterated by Denudation. Great Faults the Result of repeated Movements. Arrangement and Direction of parallel Folds of Strata. ”