VOLCANIC ROCKS OF FISH LAKE PLATEAU. 



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a great aggregate thickness of trachytes, alternating with augitic andesites 

 and some dolerites. The intercalary relations of the trachytes with the augitic 

 sheets is conspicuously marked, as is also the transition from hornblendic 

 trachytes near the base of the exposures to argilloid, granitoid, and even 

 hyaline trachytes at the summit of the exposures. These older trachytes 

 are dark gray, sometimes with a greenish or olive tinge, suggestive of the 

 andesitic group, but retaining a predominance of the trachytic characters. 

 Among them are found what appear to be augitic trachytes, but they have 

 not yet been studied very critically, and tliey differ notably in their macro- 

 scopic facies from the more abundant and voluminous augitic trachytes 

 lying at lower levels around Salina Canon. About the middle, or a httle 

 below the middle, of the mass are found very heavy beds of argilloid 

 trachyte. Throughout the nortliern part of the district there is no single 

 variety of rock which occurs in such massive beds or with such frequency. 

 Its texture and habit are strongly individualized and peculiar. It varies 

 somewhat in color, ranging from dull red to a dark purplish hue; in 

 fact, having the same range of colors as common clay-slate. It is soon 

 recognized in the great walls of the plateaus by its color, especially at 

 sunset, when the cliff faces the west, or in the morning when the cliff 

 faces the east. At such times the color characters come out strong and 

 clear, and the greater thickness of the beds also adds confidence to the 

 recognition. Higher up many varieties of light-gray trachyte are found, 

 belonging to the sanidin-trachyte group. Many of these have the char- 

 acters of clinkstone (not phonolite), being resonant and foliated in a peculiar 

 manner. Some of the sheets are broken up by a system of cleavage joints 

 into regular tiles an inch or two in thickness, and having from one to three 

 square feet of surface in the broader faces. In other sheets the cleav- 

 age, though conspicuous, is not so regular. Upon the extreme summit of 

 Fish Lake Plateau is a small remnant of an ancient coulee, which was once 

 no doubt of large proportions. It is of the granitoid variety, and all that 

 now remains are some large blocks (as large as cottages), looking like huge 

 bowlders clustered together. Several of these are poised upon smaller 

 blocks, and during a keen blast of hail and snow I had once an occasion to 

 feel grateful for the shelter afforded lue when I crept beneath one large 



