The results of this long-continued action are of the most startling kind. I have referred to the phenomenon of crag and tail as perhaps the most readily observable. We must not fail to remember that the crag which now stands up so prominently above the level of the surrounding country, at one period lay buried beneath an accumulation of sandstone, shale, or other strata, all of which have been carried away, so as to leave the harder rock in bold relief, with a portion of the less coherent strata sloping as a long tail from its eastern side. The crag, too, is often breached in many places, worn down at one end, rounded on the summit, and sometimes well-nigh ground away altogether, whilst in front there is invariably a deep hollow scooped out by the current when arrested by the abrupt cliff. In Fig. 2, _a_ represents a crag of greenstone worn away and bared of the shales which once covered it; _b_, the sloping "tail" of softer strata, protected from abrasion by the resistance of the trap-rock, and covered by a deep layer of drift, _d_; _c_ marks the hollow on the west side of the crag. [Illustration: Fig. 2 "Crag and tail."] But when we come to measure the actual amount of material that has been carried away, we are lost in conjecture as to the vastness of the time which such a process must have occupied. For instance, the coal-bearing strata of Mid-Lothian must at one period have been connected with those of Linlithgow and Stirling. At a subsequent date, the western area subsided to form the Stirlingshire coal-basin, and the eastern area, in like manner, sank down to form the coal-basin of Mid-Lothian, while the intermediate portion stretched from east to west as a great arch, or, as it is termed geologically, an _anticlinal axis_. Now, the whole of this arch has been worn away, not a vestige of it remains, and yet its upper or coal-bearing part was fully 3000 feet thick.[12] [Footnote 12: This remarkable example of denudation was first described by Mr. M'Laren, in his _Sketch of the Geology of Fife and the Lothians_, a work in which the author showed himself to be in advance of the science of his time.] Let us take a small portion of this district, and endeavour to calculate the amount of matter thus removed. The Pentland hills form a chain stretching from near Edinburgh for some fourteen miles southward, and having an average breadth of about two miles and a half. They are formed chiefly of felspathic trap-rocks, resting upon and interstratified with conglomerate apparently of Old Red age, which in turn lies upon vertical Silurian slates. Before the Carboniferous strata were thrown down by successive _faults_, they must have covered these hills completely to a depth of not less than 6000 feet.[13] From this small area, therefore, stratified sandstones, shales, limestones, and coal, must have been removed to the enormous extent of one billion, eight hundred and fifty-four thousand, four hundred and sixty-four millions of cubic feet.