Page:Popular Science Monthly Volume 7.djvu/698

678 great depth, and hence it was reasoned that the arid period that preceded the Glacial Epoch was many times longer than that which has followed it.

Even during the Glacial Epoch, Mr. Gilbert considers that "the Atlantic slope, and the region of the Great Basin, were contrasted in climate, just as now. The general causes that covered the humid East with a mantle of ice, sufficed, in the arid West, only to flood the valleys with fresh water, and send a few ice-streams down the highest mountain-gorges."

—The summit of the Kaibab Plateau is more than 6,000 feet above the river, and I have already mentioned that the summit of the plateau is also the summit of rocks of the Carboniferous Age. These beds are about 3,500 feet in thickness, and beneath them we have 1,000 feet of conformable rocks of undetermined age. This gives us 4,500 feet, from the summit of the plateau down to the non-conformable beds. Still beneath these we have 1,500 feet, so that we have more than 1,500 feet of other rocks exposed in the depths of the Grand Cañon. Standing on some rock, which has fallen from the wall into the river—a rock so large that its top lies above the water—and looking overhead, we see a thousand feet of crystalline schists, with dikes of greenstone, and dikes and beds of granite. Heretofore we have given the general name granite to this group of rocks; still, above them we can see beds of hard, vitreous sandstone of many colors, but chiefly dark red. This group of rocks adds but little more than 500 feet to the height of the walls, and yet the beds are 10,000 feet in thickness. How can this be? The beds themselves are non-conformable with the overlying Carboniferous rocks; that is, the Carboniferous rocks are spread over their upturned edges.

In the figure (p. 672) we have a section of the rocks of the Grand Cañon. A, A, represents the granite; a, a, dikes and eruptive beds; B, B, these non-conformable rocks. It will be seen that the beds incline to the right. The horizontal beds above, C, C, are rocks of Carboniferous Age, with underlying conformable beds. The distance along the wall marked by the line x, y, is the only part of its height represented by these rocks, but the beds are inclined, and their thickness must be measured by determining the thickness of each bed. This is done by measuring the several beds along lines normal to the planes of stratification; and, in this manner, we find them to be 10,000 feet in thickness.

Doubtless, at some time before the Carboniferous rocks C, C, were formed, the beds B, B, extended off to the left, but between the periods of deposition of the two series, B, B, and C, C, there was a period of erosion. The beds themselves are records of the invasion