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Show VOLCANIC BOCKS OF FISH LAKE PLATEAU. 265 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 they differ notably in their macroscopic facies from the more abundant and voluminous augitic trachytes lying at lower levels around Salina Gallon. About the middle, or a little below the middle, of the mass are found very heavy beds of argilloid trachyte. Throughout the northern 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 characters 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 cleavage, 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 \hh, shelter afforded me when I crept beneath one large |