I 921 ] Clark: Lower and Middle Cambrian Formations 



3 



GEOLOGICAL RELATIONS 



The sedimentary formations in Bristol Mountain rest on pre-Cam- 

 brian granite and dip at a high angle to the east, passing under 

 Tertiary eruptive rocks. They comprise quartzite, sandstone, shale, 

 conglomerate, and limestone in a conformable series. A repetition of 

 this series across the strike of the formations has resulted from local 

 faulting, which is very common ; and step faulting is well exemplified 

 at various localities. A good example of the effect of step faulting is 

 shown in the western end of the exposure of sedimentary rocks in the 

 Ship Mountains and also in the southern end of Bristol Mountain. 



The Invasion of Marine Waters. — The invasion of marine waters 

 in early Cambrian time is well known in the Cordilleran geosyncline. 

 It began early in the period, entering the continent at about the lati- 

 tude of the present Gulf of California, passing inward across southern 

 California and across Nevada toward the north for several hundred 

 miles. This sea probably connected with the Arctic Ocean. This por- 

 tion of the continent transgressed by the early Cambrian sea was far 

 advanced in the geomorphic cycle, the country being a peneplain. 

 The surface of the basal complex, which is granite in the area studied, 

 is fresh and is entirely devoid of any regolith. The sea probably 

 advanced slowly over southern California and the wave action removed 

 the regolith, depositing the quartz as sandstone near- the strand line 

 on the smooth wave cut surface of the fresh granite complex ; while 

 the remaining granite materials were pulverized to fine sediments, 

 carried farther out to sea and deposited as mud overlying the sands. 



Pre-Cambrian Granite. — The pre-Cambrian granite is the oldest 

 rock in the region and underlies the sedimentary formations. It out- 

 crops along the western border of Bristol Mountain where overlain by 

 Paleozoic strata, but at other places where not covered by Tertiary 

 lavas it forms a considerable part of the range. 



According to Cavins, 4 the principal minerals of the granite are 

 crystals of orthoclase which are an inch or more in length, quartz, and 

 brown biotite. The quartz and feldspar constitute about 90 per cent 

 of the rock, being present in the proportion of about one to two. 



Lower Cambrian. — The lower Cambrian is composed of a thick bed 

 of quartzite at the base, resting on a worn surface of the pre-Cambrian 



4 Bachelor's thesis, University of California, 1915. 



