ROCKS OF THE PRE-CAMBRIAN COMPLEX 231 



diorite-gneiss. When any two of these gneisses are in contact the gneissic 

 structure of the one is found to be parallel to that of the other. 



THE SCHISTS 



The schists always appear to overlie the gneisses. They are of two 

 kinds. One has the composition of a granite and shows under the micro- 

 scope strong evidence of shearing, the feldspar and quartz grains being 

 crushed and faulted and largely reduced to minute granules. In this 

 schistose granulated groundmass large grains of quartz, feldspar, and 

 muscovite form kernels or augen, around which the lines of granules and 

 the lines of muscovite of presumably secondary origin curve. Such a rock 

 may be designated a granite-aug en-schist. The other type of schist 

 weathers a brown color, strongly resembling sedimentary limestone. It 

 often contains streaks and augen of the white granite, as shown by 

 figure 2, plate 7. On a large scale the same thing may be seen at many 

 points, but nowhere better than in the ravine which leads up to the Great 

 Gulch mine, where the streaks of granite are often several inches in 

 diameter, producing the impression of intrusive sheets in the darker 

 schist. The rocks are here greatly plicated, and as the streaks of granite 

 are involved in this plication, it is evident that the plication occurred 

 after the intrusion. Some of the more massive occurrences of this type 

 of schist show no augen to the unaided eye. At the west base of the 

 ridge of Lower Cambrian rocks, with an altitude of 8,400 feet, which 

 lies about 3 miles north of Eed mountain, immediately underlying the 

 Lower Cambrian limestone, are certain dark rocks resembling slates, in 

 which are dikelike streaks of the coarse white granite at one or two 

 points. These slate-like rocks were at first supposed to be a part of the 

 Cambrian series, but the microscope shows them to be of essentially the 

 same composition as the schists above described, and hence they are 

 mapped as a part of the complex. While these schists vary in macroscopic 

 appearance, under the microscope these differences are less striking. The 

 fine grained schists and slaty rocks are found to always contain grains 

 or augen of feldspar or quartz, or both, and often of secondary minerals, 

 including an epidote-like mineral, in a groundmass of minute grains of 

 carbonate of lime. The fine grained portions of the coarser calcareous 

 augen-schists, represented by figure 1, plate 7, are an exact facsimile 

 in some cases, as seen under the microscope, of the fine grained schists, 

 which, when massive, so strongly resemble sedimentary limestone on ex- 

 posed surfaces (see figure 2, plate 7). 



Nearly all of the thin-sections show evidence of strong shearing or 

 mashing, the original grains being thoroughly crushed and arranged in 



