234 H. W. TURNER GEOLOGY OF THE SILVER PEAK QUADRANGLE 



the overlying Cambrian sediments. Where the Cambrian rocks have been 

 disturbed, the pre-Cambrian rocks have likewise been disturbed; where 

 they lie nearly horizontally, the same is true of the underlying rocks. 



The crushing and recrystallization of the rocks of the pre-Cambrian 

 appears thus to have been effected without tilting or folding, and pre- 

 sumably occurred before the deposition of the Lower Cambrian sedi- 

 ments, for in these there is little evidence of a schistose structure or of 

 recrystallization, the slates or shales often containing recognizable- Olenel- 

 lus and other fossils not far from the contact of the pre-Cambrian with 

 the Cambrian. This would likewise suggest that the white granite in- 

 volved with the augen-gneisses is older than the Cambrian, as otherwise 

 its intrusion would have produced contact metamorphic effects on the 

 Cambrian sediments, and that the dikes of white granite which at some 

 points intrude the basal dolomitic rocks are of later origin than the 

 gneisses of the complex and of the same age as the coarse pegmatite dikes, 

 from which they differ but little in chemical composition. How hori- 

 zontal gneissic and schistose structures may be produced is difficult to 

 explain, but probably they developed under a great superincumbent mass 

 of beds now eroded. Much finer examples of flat tying gneisses are 

 described by Dr Frank D. Adams 8 in Canada. This area of gneisses is 

 referred by Doctor Adams to the Grenville series. In these gneisses are 

 interpolated occasional bands of crystalline limestone and of quartzite. 

 Over an area of at least 750 square miles the gneisses lie quite flat, or 

 pt most dip at an angle of 30 degrees. In one of the illustrations in 

 Doctor Adams' paper (plate III, p. 13-/) a bluff of these horizontal 

 gneisses is depicted which might readily be mistaken for undisturbed 

 sedimentary rocks at a little distance. 



Dr George M. Dawson, 9 in his presidential address before the Geolog- 

 ical Society of America, in describing the Shuswap series of the Eocky 

 mountains in Canada, indicates a relation between the Archean and 

 Cambrian similar to that above described on Mineral ridge. He writes : 



"A distinct tendency to parallelism of the strata of foliation with adjacent 

 borders of the Cambrian system has, moreover, also been noted in a number of 

 cases. This might imply that the foliation was largely produced at a time 

 later than the Cambrian, but the materials of some of the Cambrian rocks show 

 that the Shuswap series must have fully assumed their crystalline character 

 before the Cambrian period, and there are other evidences of their extensive 

 pre-Cambrian erosion. It seems, therefore, probable that the foliation of the 



8 Geological Survey of Canada ; Report of progress, 1895, vol. viii. Report by Frank 

 D. Adams on the geology of a portion of the Laurentian area lying to the north of the 

 island of Montreal, pp. 11-/. 



9 Bull. Geol. Soc. Am., vol. 12, 1901, pp. 57-92. 



