58 GEOLOGY OP THE HIGH PLATEAUS. 



in the deepest ravines of the Awapa, near the Aquarius, where profound 

 excavations, near the great faults, have disclosed them beneath nearly 3,000 

 feet of trachytes 



A qiiestion has been carefully considered, without reaching a positive 

 conclusion, whether the tufaceous beds already spoken of may not have 

 been derived from the waste of these propylites. The tufas are wholly 

 water-laid beds. Their ordinary aspect is well represented in Heliotypes V 

 and VI. The stratification has all of the mechanical characters of ordinary 

 arenaceous beds. In numerous places the tufas are seen to pass horizon- 

 tally by gradual transition into ordinary arenaceous shales, made up wholly 

 of materials derived from the decay of non-eruptive rocks. The propy- 

 lites alone of all the massive rocks seem to have sufficient antiquity to have 

 supplied the material for these deposits, and the only question seems to be 

 whether these came from the visible propylites or some unknown volcanics 

 of still greater age. The tufas have been carefully studied with the micro- 

 scope in the hope of settling the question, but no solution has been reached. 

 They contain large quantities of quartz and feldspar, which are often 

 epigenetic, and the remaining contents are so much decayed that their 

 original characters are obliterated. But although the antecedence of the 

 propylites to the tufas cannot be proven, it may at least be said that there 

 is no fact now known which forbids such a conclusion. More than that, 

 the inference has some slight preponderance of probability in its favor. 



The hornblendic andesites succeeded the propylites with apparently a 

 long interval between them. They were erupted from the same localities 

 or from vents in the immediate vicinity. The mass of these rocks now 

 exposed is greater than that of the propylites, and the lavas are consider- 

 ably more varied in texture and appearance. Their principal locus seems 

 to have been in the southern part of the Sevier Plateau, though the masses 

 revealed in the northern part of the same uplift are but little inferior. The 

 outbreaks were in massive sheets, which stretched far to the eastward and 

 southeastward, spreading- out over large areas and piling up mountainous 

 masses. It is not, however, the quantity now exposed which gives us the 

 rial clue to the magnitude of the andesitic extravasations, but rather the 

 glial bulk el' tin- conglomerates derived from their ruins. The andesitu.s, 



