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 Markagunt; but for some reason I was more 

 profoundly impressed with it in the Awapa than elsewhere, though, pos- 

 sibly, it may be no greater in the latter than in the former. But assur- 

 edly 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 indi- 

 vidual 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 flow- 

 ing 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- 



