678 THE POPULAR SCIENCE MONTHLY. 



great depth, and hence it was reasoned that the arid period that pre- 

 ceded 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 val- 

 leys with fresh water, and send a few ice-streams down the highest 

 mountain-gorges." ' 



Records of More Ancient Lands. 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 Canon. 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 Car- 

 boniferous 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 

 Canon. A, A, represents the granite ; or, a, dikes and eruptive beds ; 

 J^, JB, 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 jb, y, is the only part of its height 

 represented by these rocks, but the beds are inclined, and their thick- 

 ness 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 H, H, extended ofi" to the left, but between the 

 periods of deposition of the two series, M, B, and C, (7, there was a 

 period of erosion. The beds themselves are records of the invasion 



'Bulletin of the Philosophical Society, Washington, forty- sixth meeting, April 

 26, 1873. 



