Sometimes, when beds have been torn asunder, one side of them has been forced up above the other; and there are even instances where one side of a mountain has been forced under the surface of the earth, while the other has remained above. Stratified beds of rock are even found which have been so completely capsized, that the layers, which were of course deposited horizontally, now stand on end, side by side, in vertical rows. I remember, after a lecture on some of these extravagances in mountain-formations, a friend said to me, not inaptly,--"One can hardly help thinking of these extraordinary contortions as a succession of frantic frolics: the mountains seem like a troop of rollicking boys, hunting one another in and out and up and down in a gigantic game of hide-and-seek." The width of the arch of a mountain depends in a great degree on the thickness and flexibility of the beds of which it is composed. There is not only a great difference in the consistency of stratified material, but every variety in the thickness of the layers, from an inch, and even less, to those measuring from ten or twenty to one hundred feet and more in depth, without marked separation of the successive beds. This is accounted for by the frequent alternations of subsidence and upheaval; the continents having tilted sometimes in one direction, sometimes in another, so that in certain localities there has been much water and large deposits, while elsewhere the water was shallow and the deposits consequently less. Thin and flexible strata have been readily lifted into a sharp, abrupt arch with narrow base, while the thick and rigid beds have been forced up more slowly in a wider arch with broader base. Table-lands are only long unbroken folds of the earth's surface, raised uniformly and in one direction. It is the same pressure from below, which, when acting with more intense force in one direction, makes a narrow and more abrupt fold, forming a mountain-ridge, but, when acting over a wider surface with equal force, produces an extensive uniform elevation. If the pressure be strong enough, it will cause cracks and dislocations at the edges of such a gigantic fold, and then we have table-lands between two mountain-chains, like the Gobi in Asia between the Altai Mountains and the Himalayas, or the table-land inclosed between the Rocky Mountains and the coast-range on the Pacific shore.