120 DESCRIPTIVE GEOLOGY. 



din. Macroscopical quartz occurs in comparatively large grains, while the 

 hornblendes, though small, are very abundant. 



It has already been mentioned that the divide between the Middle and 

 North Parks is composed of eruptive rocks, which stretch between the two 

 great Archfean ranges. Although the summits and the greater part of the 

 northern slopes are thus formed, this east and west ridge does not exclu- 

 sively consist of eruptive material; the foot-hills in many places being made 

 up of long ridges of Cretaceous sandstones, which have been elevated into 

 their present position by the Tertiary volcanic outbursts. These volcanic 

 rocks consist of both trachytes and basalts, and occur both as large, massive 

 bodies, and as narrow dikes, penetrating the Mesozoic strata. The trachytes 

 occupy by far the most considerable area, and have influenced to a greater 

 extent the present form and outline of the ridge. Parkview Peak, the highest 

 point along the divide, stands out prominently from the surrounding country, 

 rising over 12,000 feet above sea-level. It is situated just south of the limit 

 of the map, near the southeast corner of the Park, one of the tributaries ol 

 Sioux Creek draining its northern slope. Its summit consists of light-colored 

 rocks, probably trachytes, which form, upon the northern side, long spurs 

 and ridges extending down to the Park. 



An instructive geological feature of Parkview Peak, and one rarely 

 met with within the area of this survey, is the occurrence of heavy beds of 

 Cretaceous sandstones, elevated by trach3^tic outbursts, and now found 

 imbedded between large masses of volcanic rocks, and in places penetrated 

 by dikes, frequently only a few feet in width, which, withstanding atmos- 

 pheric agencies better than the enclosed sandstones, rise above the latter 

 like abrupt walls, with a general strike of east and west. The sandstone 

 beds appear quite hard, and somewhat metamorphosed, showing the effect 

 of the intrusion of adjacent trachytes. All the observed beds dip to the 

 northward, Avith varying angles, but their inclination is of little importance, 

 owing to the irregular manner in which the trachytes have reached the sur- 

 face. Of the age of the sandstones, no definite proof was obtained, although 

 it would seem more than probable that tliey belong to the Colorado 

 group, as very similar beds are found to the westward uj)turned by tra- 

 chytic and basaltic outbreaks, whose age is undoubtedly that of the Colo- 



