272 GEOLOGY OF THE UIGn PLATEAUS. 



" at a distance of 8 miles south. The mass of lavas appears to be of great 

 thickness, and the sedimentary beds are not seen beneath them until we 

 approach the vicinity of Thousand Lake Mountain. The eruptions were of 

 the most massive character, being in some instances more than a hundred 

 and twenty feet thick, and presenting ledges of rock several miles in length. 

 The eruptive materials are of the same general character as those ob- 

 served in the Fish Lake Plateau. They are mostly trachytic, with subor- 

 dinate though considerable masses of augitic andesite and dolerite interca- 

 lating. All of the trachytes have a dark, somber appearance, and belong to 

 both of the divisions of that group. The older varieties are hornblendic, 

 with considerable plagioclase, and among them are also found augitic tra- 

 chytes. The younger members are chiefly of the argilloid varieties, and, as 

 elsewhere, they occur in immense beds. 



THE AWAPA PLATEAU. 



The Awapa and Aquarius Plateaus have not been studied in detail, and 

 my knowledge of them is such only as has been derived from a few rapid 

 transits across the former in different places, and about three weeks spent 

 upon the flanks of the latter. Their area is very great, and, in order to 

 acquire sufficient data to give any detailed account of them, much more 

 labor and travel is necessary. I have been much indebted to some notes 

 prepared by Mr. E. E. Howell, whose observations have supplemented my 

 own in some very important particulars, and have prepared the way to the 

 determination of many points ; especially those relating to the stratigraphy 

 and structure of the Aquarius Plateau. 



The separation of the Awapa from the Fish Lake Plateau is probably 

 more justifiable on the ground of convenience of discussion than of reality, 

 for the latter passes directly into the former. A sudden descent across a 

 steep slope brings us from one to the other. For a short space the lake 

 may be regarded as a natural barrier, but east of the lake it is not practi- 

 cable to say where the one ends and the other begins. The Fish Lake 

 table has an altitude of more than 11,000 feet, and where its southern end 

 drops down upon the Awapa the altitude of the latter is less than 9,000 

 feet ; and generally the altitude of the Awapa is the least of the several 



