320 GEOLOGICAL SURVEY OF THE TERRITORIES. 



fine-grained ash. One feature 1 will not omit here, as it is constant in 

 the entire western part of section a.- The long, stretched plateaus, 

 having for a basis either andesite or tuff, are almost invariably crowned 

 with a highly vesicular flow of the lava, showing andesitic composition. 

 The color is dark, almost black ; the paste very even in texture, only 

 a few mineral segregations have taken place. In most cases, the vesicular 

 cavities have an approximately oval shape ; but some were found 

 in which these cavities were elongated to the extent of over half an inch, 

 lirobably owing to the fact that the material was moved, or moved 

 on farther, after having partially hardened. The cavities are frequently 

 filled with carbonates of lime. Oligoclase and sanidite are contained as 

 small crystals in the paste. Hornblende and mica are rare. Magnetite 

 occurs quite abundantly. It seems, from the position this material 

 has, that it must belong to the andesite, and, therefore, I refer it to that 

 order, supposing it to be the youngest member. Between the older 

 andesite and the one in question a large mass of material is deposited — 

 breccias, tuff's, and ash — this vesicular lava covering them all ; but I did 

 not observe a single instance where true trachyte was covered by it, 

 Eeference to the section may show the position of these formations. 

 Station 56 is a prominent point, sloping off gently to the south, and 

 having a steep slope on the northern side. It is composed of andesite 

 of a dark color, whence'its name, "Black" Mountain. The paste is 

 fine-grained and dense, not vesicular; small crystals of oligoclase, sani- 

 dite, and brown mica occur throughout. Hornblende is rare ; magnetite 

 comparatively abundant. Although the color of the rock is dark-gray, 

 the weathered surface is always reddish-brown. Proceeding toward 

 the southwest, we find the vesicular andesitic lava overlying this ande- 

 site, but a cut which exposes about 120 feet vertically shows that the 

 vesicular material also rests immediately upon a very thick bed, probably 

 over 200 feet of volcanic ash. This ash is of a white, light-greenish, light- 

 brown to pink color, very soft, so that it will crumble in the hand, or 

 may be cut with a knife ; it weathers in very picturesque groups, that 

 are well set oft' by the variation of color. Farther down, along the 

 slope, the tuff sets in, probably underlying in part the ash. Wherever 

 this tuff outcrops, it forms grotesque groups of rocks of a light-grayish 

 or yellowish color. After that the character changes ; several small 

 hills are formed by rhyolite along the base of the mountain, forming to 

 a considerable distance small bluffs i)arallel to it, and partly overlying 

 the tuff, partly the Carboniferous sandstone. It is at first red, of jas- 

 pery texture, with crystals of sanidite and oligoclase, aJso scattering 

 particles of black mica. Free quartz can be noticed, but there is not 

 much of it. On the second bluff, the rhyolite is white ; the paste is not 

 quite so compact as that of the red, but the minerals contained in it are 

 identical. In some of the rhyolites and tuffs it may be noticed that the 

 sanidite has a blue light, like thelabradorite, with which it may be readily 

 confounded. On the west side of the sedimentary formations, tracho- 

 rheites are also found. In the main, they seem to consist in tuffs with an 

 andesitic nucleus. True trachyte is not wanting in either the section 

 east or that west of the Carboniferous and Silurian. Its appearance is 

 more local, however, and it ajiparautly did not flow to such an extent 

 as the rhyolites or tuffs. Station OS is located on such trachyte. Iso- 

 lated patches of granite are distributed throughout the trachorheitic 

 area just mentioned ; but the granite that appears is rarely the well- 

 known coarse-grained red variety surrounding all these volcanic forma- 

 tions ; it is always more or less metamorphosed. Stations 51 and 52 

 are located on a narrow strip of trachyte running down from the north- 



