2. The _sedimentary rocks_ have also been subject to great denudation; and we often have, in what is left, some indications of how much has been removed. One of these indications consists in the now level surface of those portions of country in which large _faults_ exist. By the excavations for coal, in England, faults have been discovered of five or six hundred feet. At the time that they were formed, the surface must have presented precipitous escarpments (as represented by the dotted lines in Fig. 50) of a height equal to the dislocation; but the whole is now reduced to a general level (_z z_), denuding causes having removed the elevated portions. The extent of _valleys_ will often give some idea of the amount of denudation to which a region has been subjected. In the north-west of Scotland there is a succession of hills of about three thousand feet in elevation, consisting, for the upper two thousand feet, of horizontal strata of old red sandstone. (Fig. 66.) We cannot conceive that these mountain masses were deposited in their present isolated form. The whole intervening spaces must have been filled with strata continuous with those by which the elevations are formed.[D] [D] "I entertain little doubt that when this loftier portion of Scotland, including the entire Highlands, first presented its broad back over the waves, the upper surface consisted exclusively, from one extremity to the other, of a continuous tract of old red sandstone; though, ere the land finally emerged, the ocean currents of ages had swept it away, all except in the lower and last raised borders, and in detached localities where it still remains, as in the pyramidal hills of Western Rosshire, to show the amazing depth to which it had once overlaid the inferior rocks."--_Miller, Old Red Sandstone_, _p. 22_. [Illustration: Fig. 66.] [Illustration: Fig. 67.] A somewhat similar instance occurs in the Connecticut river sandstone, in the central part of Massachusetts. The following figure (Fig. 67) represents two mountains of 1 the sandstone, between which the Connecticut river flows. The dotted lines indicate a depth of one thousand feet of the rock which has been swept away. It is also thought that a bed of equal depth has been removed from this section southward, through the State of Connecticut, to the sea-coast. 3. _Valleys_, and even many of the larger valleys, are produced by the wearing down of the surface. The lower portion of the Connecticut valley is one of denudation, though in its upper part it is a valley of elevation, resulting from the upheaval of the Green and White Mountains. The water-courses from the mountains are transverse to the direction of the ranges, and generally consist of valleys of denudation. These valleys were no doubt originally fractures, produced while the mountains were rising. The fractures have been subsequently widened by denudation into valleys.