SUCCESSION OP ERUPTIONS IN THE MAKKAGUNT. 197 



rough surface ; but this trachyte breaks with an exceedingly jagged, angu- 

 lar, and in-egular fracture, so tliat it is impossible to hammer out a neat and 

 shapely specimen. The grandest masses of trachyte, not only in the Markd- 

 gunt but in the other plateaus, consist of this variety. It lies in immense 

 beds, often two or three hundred feet in thickness, spreading out over many 

 square miles with remarkable regularity and homogeneity. In the MarkA- 

 gunt it forms mesa-like platforms, ending in low precipices, where the shal- 

 low canons and ravines have cut into it. It breaks up or rather crumbles 

 with unusual facility for an eruptive rock, producing a coarse gravel, which 

 floors the ravines below. This rock is so distinct in its characters that it 

 seems almost to justify a separate name, but I shall content myself with a 

 purely descriptive designation, and call it argilloid trachyte. 



The augitic varieties of trachyte are found in sheets, which are usually 

 much thinner and cover smaller areas, though the number of them is much 

 greater. The total bulk is less than that of the argilloid variety, though 

 absolutely it is very great. 



The rhyolites are the third group of eruptives found in the Markdgunt. 

 They are seen in large masses along the very highest part of the plateau, 

 from the crest of which they poured out in massive sheets. They are 

 probably as ancient as the older liparitic masses of the Tushar, but always 

 overlie the trachytes whenever they are in contact with them. They belong 

 altogether to the liparitic sub-group, with an abundance of porphyritic crys- 

 tals of feldspar and quartz. None of those hyaline fluent rhyolites which 

 characterize the northern Tushar are seen here. Although their volume is 

 very great, it is far less than that of the trachytes, and the areas which they 

 cover are much smaller. 



The fourth group is the basaltic. Among the High Plateaus the Mai'- 

 kdgunt and Tushar alone present extensive outpours of rocks of this class. 

 A few small eruptions are found in the eastern plateaus and notably in the 

 intervening valleys, but they are not comparable in extent to those of the 

 Markagiint. Here they are confined to the southern half of the plateau. 

 A little south of the center is a large tract in which are still preserved 

 remnants of a considei-able number of basaltic craters, though so much 

 degraded that they are not immediately recognized. ^J hey fonn a large 



