204 GEORGE W. STOSE 



GEOLOGY 



The rocks in this area are largely concealed by the sandstone 

 debris which covers the mountain tops as well as the valleys and 

 slopes. Their character, thickness, and relation are therefore not 

 readily determined. The structure is also complicated by schis- 

 tosity and jointing which exist in all these rocks. The mountains 

 are composed of Georgian (Lower Cambrian) quartzites, sand- 

 stones, and shales, and older igneous rocks; the adjacent portions 

 of the Valley of Cambrian and Ordovician limestones and shales. 



STRATIGRAPHY 



Old volcanics. — The basement rocks exposed in the area are 

 ancient volcanic rocks, greenstone and altered rhyolite, which 

 underlie the basal Cambrian uncomformably. They occupy the 

 plateau-Uke tract overlooked by higher peaks in the center of the 

 mountain area shown on the map and in the extreme southeast 

 corner, and are extensively developed to the eastward. The vol- 

 canic origin of these ancient rocks is clearly shown by flow banding, 

 amygdaloidal structure, and spherulites, as described by Williams' 

 and Bascom.^ The greenstones are sheared dense rock, veined 

 with asbestos and chlorite. The original structure is seldom pre- 

 served, but the rock is apparently an altered basalt. The rhyo- 

 litic rocks are of purple and red tints, often porphyritic and fre- 

 quently banded by flow structure or spherulitic streaks. The 

 rhyolitic rocks predominate in this area, and apparently overhe 

 the greenstone, for the basal Cambrian sediments are composed 

 largely of rhyolitic fragments and not of basaltic detritus, as would 

 be the case if the greenstone were younger and had been eroded 

 from most of the area. 



Basal sandstones. — Overlying these softer rocks are about 4,500 

 feet of sandstone, quartzite, and shale of Georgian (Lower Cam- 

 brian) age. The basal beds, forming the higher and more rugged 

 portions of the mountains, are composed of coarse, purple and yel- 

 lowish banded sandstones, fine conglomerate, and arkose, with white 



1 American Journal of Science, Third Series, Vol. XLIV, pp. 482-96. 



2 Journal of Geology, Vol. I, pp. 813-32, and Bulletin of the U. S. Geological 

 Survey, No. 136. 



