NORTHERN focus of eruptions. 233 



say. To the westward they are cut off in the great wall which faces 

 Sevier Valley with an altitude of nearly 6,000 feet above the river. To 

 the east they are likewise cut off by the oblique valley, though they 

 reappear at lower altitudes on the other side, and are instantly lost again 

 under soil and waste, but evidently descend into Grass Valley, and may 

 commingle with the equally grand floods emanating from the Fish Lake 

 Plateau to the eastward. But south and north they are displayed in im- 

 mense volume. Those which flowed north and northeast are spread out 

 in the vicinity of Salina Canon and one great coulee stretched beyond the 

 canon, which now cuts off a portion of it, leaving it as an outlier. Large 

 portions of these old lavas have been swept away. The mauvaises terres 

 south of Salina village were once covered with it. Standing prominent 

 among these bad lands is a conical butte-like mountain of singularly per- 

 fect form. It is a remnant left by circum denudation, and upon its summit 

 is a "tip" or cap about 250 feet thick, consisting of this same lava reposing 

 upon the sedimentary strata, out of which the peak has been carved in 

 cameo. This mountain is called the Black Cap. The augitic trachyte,* of 

 which its summit apparently forms a remnant, is the same as that which 

 extends across the Salina Canon. This flow reached a distance of 30 miles 

 from its source South of the canon and nearer the source sheets of argil- 

 loid trachyte rest upon the augitic and hornblendic, and heavy beds of con- 

 glomerate derived from the ruins of both kinds of rock are interspersed. 

 To the northeastward, extending as far as 25 miles, similar aggregates of 

 massive superposed -coulees are displayed, having a thickness of nearly a 

 thousand feet and increasing in bulk as we approach the Sevier Plateau. 

 The hornblendic trachytes are in the larger proportion, but the lighter gray 

 trachytes, and especially the ' argilloid ' varieties, are almost as voluminous. 

 They are much degraded by erosion, and several fine canons have been cut, 

 ramifying into broader ravines, with big rough swelling hills between them. 



* This rock is a conspicuous one. It has many crystals ol sanidin, but the less conspicuous plagio- 

 clase is very abundant. The line is difficult to draw — perhaps impossible — between some andesites and 

 augitic trachytes. The texture is sometimes the only basis of a distinction, and this should be used 

 with great caution, and never without reservations. Still the textures of the two groups are usually 

 distinct and characteristic, and the rock assumes in most cases the one aspect or the other even when 

 the mineralogical constitution is doubtful. In the very few cases where there is no means of forming 

 a decided distinction it would seem as if the old term " traehy dolerite " might be useful. It has the 

 advantage at least of being non-committal. 



