230 GEOLOGY OF THE HIGH PLATEAUS. 



of conglomerate, formed of their fragments, were accumulated and are here 

 and there brought to light where erosion has deeply excavated the still 

 grander masses of subsequent lavas overlying them So completely were 

 these most ancient rocks overwhelmed, that erosion has only revealed a 

 very small portion of them and left us to conjecture what may be the 

 extent of those portions now concealed. It is not improbable that the 

 clastic beds, formed of the waste of volcanic rocks, and which underlie the 

 great lava caps of the plateaus and in turn rest upon the Bitter Creek and 

 Green River beds, may have derived their sands and clays from the decom- 

 position of some of these propylitic masses. 



These ancient eruptions are succeeded by those of a middle epoch, 

 lying across the surface of an eroded country, which they overwhelmed. 

 These second lavas are much less chaotic in their arrangement and much 

 less affected by erosion during the intervals between the eruption of succes- 

 sive floods. They are, therefore, more intelligible, and some idea of their 

 sequences has been obtained, though less definite than is desirable, because 

 the exposures are so partial and so much obscured by debris and soil. 

 These outpours were upon a very large scale, the masses being often several 

 hundred feet in thickness and spreading out over large areas. The lower 

 masses are andesitic and show but little variety. They all belong to the 

 hornblendic group and are characterized by triclinic feldspar, with a mode- 

 rate proportion of hornblende, with some augite and magnetite, and are 

 very compact and rather fine-grained. Higher up, these give place to 

 coarse-grained trachytes, with both monoclinic and triclinic feldspars and 

 abundant hornblende. These occasionally intercalate with sheets of doler- 

 ite. Still higher, a totally distinct group of trachytes is found. They con- 

 sist largely of the avgilloid variety — a fine-grained, highly ferritic, reddish 

 paste, holding porphyritic crystals of opaque monoclinic feldspar. There is 

 probably no eruptive rock within the district more abundant. It forms the 

 summit of the series of middle-aged eruptions in many localities. Very 

 nearly coeval with it is a group of trachytes, having an appearance faintly 

 resembling a fine-grained S3*enite, though not by any means wholly crys- 

 talline. It varies in color from iron gray to light gray. It shows a tendency 

 to break up into slabs or tiles from an inch to four or five inches thick, the 



