The very different levels at which the separated parts of the same strata are found on the different sides of the fissure, in some faults, is truly astonishing. One of the most celebrated in England is that called the “ninety-fathom dike,” in the coal-field of Newcastle. This name has been given to it, because the same beds are ninety fathoms (540 feet) lower on the northern than they are on the southern side. The fissure has been filled by a body of sand, which is now in the state of sandstone, and is called the dike, which is sometimes very narrow, but in other places more than twenty yards wide.[[9]] The walls of the fissure are scored by grooves, such as would have been produced if the broken ends of the rock had been rubbed along the plane of the fault.[[10]] In the Tynedale and Craven faults, in the north of England, the vertical displacement is still greater, and the fracture has extended in a horizontal direction for a distance of thirty miles or more.
Great Faults the Result of Repeated Movements.—It must not, however, be supposed that faults generally consist of single linear rents; there are usually a number of faults springing off from the main one, and sometimes a long strip of country seems broken up into fragments by sets of parallel and connecting transverse faults. 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. Some geologists have considered it necessary to imagine that the upward or downward movement in these cases was accomplished at a single stroke, and not by a series of sudden but interrupted movements. They appear to have derived this idea from a notion that the grooved walls have merely been rubbed in one direction, which is far from being a constant phenomenon. Not only are some sets of striæ not parallel to others, but the clay and rubbish between the walls, when squeezed or rubbed, have been streaked in different directions, the grooves which the harder minerals have impressed on the softer being frequently curved and irregular.
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 Fig. 76, p. 89, and the same remarkable fact is well exemplified in every coal-field which has been extensively worked. It is in such districts that the former relation of the beds which have been shifted is determinable with great accuracy. Thus in the coal-field of Ashby de la Zouch, in Leicestershire (see Fig. 77), a fault occurs, on one side of which the coal-beds a, b, c, d must once have risen to the height of 500 feet above the corresponding beds on the other side. But the uplifted strata do not stand up 500 feet above the general surface; on the contrary, the outline of the country, as expressed by the line z z, is uniformly undulating, without any break, and the mass indicated by the dotted outline must have been washed away.[[11]]
The student may refer to Mr. Hull’s measurement of faults, observed in the Lancashire coal-field, where the vertical displacement has amounted to thousands of feet, and yet where all the superficial inequalities which must have resulted from such movements have been obliterated by subsequent denudation. In the same memoir proofs are afforded of there having been two periods of vertical movement in the same fault—one, for example, before, and another after, the Triassic epoch.[[12]]
The shifting of the beds by faults is often intimately connected with those same foldings which constitute the anticlinal and synclinal axes before alluded to, and there is no doubt that the subterranean causes of both forms of disturbance are to a great extent the same. A fault in Virginia, believed to imply a displacement of several thousand feet, has been traced for more than eighty miles in the same direction as the foldings of the Appalachian chain.[[13]] 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; whereas there are no grounds for believing that, during the last 3000 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.
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. But every volcano, by the intermittent action of the steam, gases, and lava evolved during an eruption, helps us to form some idea of the manner in which such operations take place. For eruptions are repeated at uncertain intervals throughout the whole or a large part of a geological period, some of the surrounding and contiguous districts remaining quite undisturbed. And in most of the instances with which we are best acquainted the emission of lava, scoria, and steam is accompanied by the uplifting of the solid crust. Thus in Vesuvius, Etna, the Madeiras, the Canary Islands, and the Azores there is evidence of marine deposits of recent and tertiary date having been elevated to the height of a thousand feet, and sometimes more, since the commencement of the volcanic explosions. There is, moreover, a general tendency in contemporaneous volcanic vents to affect a linear arrangement, extending in some instances, as in the Andes or the Indian Archipelago, to distances equalling half the circumference of the globe. Where volcanic heat, therefore, operates at such a depth as not to obtain vent at the surface, in the form of an eruption, it may nevertheless be conceived to give rise to upheavals, foldings, and faults in certain linear tracts. And marine denudation, to be treated of in the next chapter, will help us to understand why that which should be the protruding portion of the faulted rocks is missing at the surface.
Arrangement and Direction of Parallel Folds of Strata.—The possible causes of the folding of strata by lateral movements have been considered in a former part of this chapter. 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 chain extends from north to south, or rather N.N.E. to S.S.W., for nearly 1500 miles, with a breadth of 50 miles, throughout which the Palæozoic strata have been so bent as to form a series of parallel anticlinal and synclinal ridges and troughs, comprising usually three or four principal and many smaller plications, some of them forming broad and gentle arches, others narrower and steeper ones, while some, where the bending has been greatest, have the position of their beds inverted, as before shown in Fig. 73, p. 87.
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°, the folds wheeling round together in the new direction and continuing to be parallel, as if they had all obeyed the same movement. The date of the movements by which the great flexures were brought about must, of course, be subsequent to the formation of the uppermost part of the coal or the newest of the bent rocks, but the disturbance must have ceased before the Triassic strata were deposited on the denuded edges of the folded beds.
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° 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.