In the course of the hundred feet or so of perpendicular depth thus exposed, he would have seen a succession of layers, apparently deposited upon one another. But as yet he would have formed a very inadequate notion of the stratification of the earth's crust. With the knowledge thus gained, however, let him now make a little excursion into Hertfordshire; we will suppose at the time when the cuttings for the Great Northern Railway were being made. When he came near Cheshunt, he would see that the London clay, which he found underlying the Thames, crops out, or disappears by the stratum coming obliquely to the surface. He would see, however, another bed of clay--the plastic clay--beneath this, which now forms the superficial stratum, and continues to do so, till he gets beyond Hertford. There this stratum crops out; and the chalk, which for some time he has seen to underlie the plastic clay, now comes to the surface. Business or pleasure calls him to Bridlington on the Yorkshire coast; and he determines to make a pedestrian tour across the diameter of England to Whitehaven. He soon recognises the chalk, which constitutes the Wolds, and rises to about 800 feet above the sea level. Below its escarpment he traces the Kimmeridge clay, the uppermost of a series of strata more than 2,000 feet in thickness, that constitute the Oolitic system--including, among others, the coralline oolite, the calcareous grit, the cornbrash, thin, but rich in fossils; the lower sandstone and coal of the Cleveland hills, the alum shale, the marlstone, and the lower lias shale. Then comes a stratum of the saliferous system or the new red sandstone, with the red marls, perhaps not much short of a thousand feet deep. Below them the observer finds the strata of the magnesian limestone formation, for nearly 400 feet, resting on the great coal formations of vast depth. Of these the coal field of the West Riding is not less than 4,000 feet in depth, and beneath it lie the millstone grit, and the mountain limestone, 2,500 feet more, the latter displayed in noble grandeur on the faces of those wall-like precipices that inclose the romantic dales of the Swale and the Ure, and that subsequently tower in magnificent altitude on the sides of Pennygant and Ingleborough. [Illustration: GEOLOGICAL SECTION OF YORKSHIRE.] On the western escarpment of the Pennine ridge, just as the traveller is entering Westmoreland, he would detect the bottom of the limestone; and here he would have an opportunity of seeing, what is rare in these parts, a stratum of the old red sandstone, lying between the former and the slaty rocks of the Cumbrian formations. And here at length, in the wild and magnificent scenery of these mountains, he sees the primitive and transition series, the greenstone, the sienite, and the granite, each of which is discernible in succession on the face of one or other of the lofty Fells of Cumberland.