654 DESCRIPTIVE GEOLOGY, 



forms the base of Crescent Peak, and is overlaid by a trachyte which has 

 a great deal of the external appearance of the andesites of this region. It 

 is a dark-gray fine-grained rock containing slender crystals of black horn- 

 blende, which show the same products of alteration as the andesite. The 

 feldspars of this trachyte are small and inconspicuous, but in them sanidin 

 seems to predominate over plagioclase. 



Near the head of Augusta Canon, and extending across the ridge 

 which separates it from Granite Point Canon, is a dike-like mass of columnar 

 augite-andesite, which is almost identical with the augite-andesite of Jacob's 

 Promontory. It is evidently a later outburst through the older andesites, 

 and presents a remarkably regular columnar structure, the columns being 

 generally very perfect pentagons, ranging in size from a few inches to a 

 foot or more in diameter. The exterior surface of these columns is covered 

 by a thin, regular coating, about one-tenth of an inch in thickness, of light- 

 green color, which is evidently an alteration product of the groundmass, as 

 the hornblendes and other crystalline ingredients of the rock which occur 

 in it are unaltered. Beneath this green layer is a still thinner layer of red- 

 dish, rusty material. The dark groundmass of the rock has, in fresh fract- 

 ure, a resinous, almost glassy lustre, and carries large tabular crystals of 

 plagioclase-feldspar and some sanidin, together with greenish decomposed 

 augites and crystals of hornblende, but no olivine. The microscope dis- 

 closes also the presence of magnetite, and in the groundmass a base of 

 brownish glass without globulitic secretions, which cements microlites of 

 plagioclase and augite. 



The same augite-andesite occurs on the western slopes of the range in 

 Antimony Canon, in thin, horizontal beds, which form the crest of the spur, 

 and cover the underlying earthy and brecciated andesites, which in turn, as 

 shown by occasional obscure outcrops, are underlaid by the older hornblende- 

 porphyries. In the head of this canon is also found an obscure outcrop of 

 green clay-slates, which are veiy much broken and decomposed. The 

 western slopes of the range south of Crescent Peak were not visited, but 

 as seen from the summits are evidently largely composed of rhy elite flows, 

 which cover so extensively the underlying rocks through the parts of the 

 range which were visited. 



