| OCR Text |
Show 274 GEOLOGY OF THE HIGH PLATEAUS. lavas gradually attenuate from west to east, or rather from the periphery of the plateau towards its central depression. The variety of the rocks displayed is truly astonishing. It seems as if two exposures rarely presented the same description of lava. The great majority of them belong to the trachytic group, and it is surprising to see what numberless changes can be rung upon materials which vary so little in their ultimate composition. This manifold variation is displayed in the Sevier Plateau and in the Markagimt; but for some reason I was more profoundly impressed with it in the Awapa than elsewhere, though, possibly, it may be no greater in the latter than in the former. But assuredly the number of distinct coulees is extremely great, and it is hard to find two precisely alike. Some of the trachytic beds are quite thin, being not more than 20 to 25 feet thick, and successions of these variegated layers are frequently met with. On the other hand, some of the grandest and most massive sheets in the High Plateaus are found here. On the north side of Rabbit Valley the plateau slope ends in a low wall about 100 to 120 feet in height and nearly 4 miles in length, which seems to be one individual sheet of argilloid trachyte. Some very grand sheets of hornblendic trachyte are also displayed hard by, having the exceedingly rough, coarse texture which is so characteristic of that variety. In the large coulees of hornblendic trachyte, and sometimes also in the granitoid variety, may be seen that rough, broken aspect of the lava suggestive of flowing in a very viscous and almost solid state, as if the whole mass were continually rending itself into fragments, as it crept along like a huge glacier, while more fluent portions from within the flood worked their way into the rifts, and there congealed. In the smaller and thinner sheets this phenomenon is not seen; but a more mobile character is indicated. The grander flows generally belong to the granitoid and argilloid varieties, while the smaller and more fluent ones are sometimes hyaline and sometimes augitic trachyte. Vitreous products also are common, and at the parting of the beds trachytic obsidian is found in abundance. No rhyolites have been detected in this plateau, but a few pitch-stones, with the trachytic character rather than the rhyolitic, were observed. This indicates a considerable range in the chemical constitution, and is accompanied with a correspond- |
