Geology of West Coast Region of United States 



The Pre-Cambrian of the West Coast. — For- 

 mations of pre-Cambrian age doubtless exist in 

 numerous localities in this region, but in most cases 

 the proof of the suspected antiquity of these strata 

 is lacking. East of the region under consideration, 

 intense and widespread metamorphism is generally 

 considered a proof of great geological age. In the 

 Pacific region such metamorphism merely indicates 

 an age antedating the intrusion of the Sierra Nevada 

 batholith. 



Patches of known pre-Cambrian occur north- 

 west of Owens Lake, where a metamorphosed series 

 appears underneath strata carrying an Olenellus 

 (lower Cambrian) fauna. Numerous areas of prob- 

 able pre-Cambrian occur in the desert mountain 

 ranges of the region adjoining Arizona, as proved 

 pre-Cambrian is found in the Grand Caiion of the 

 Colorado, and neighboring areas. All other com- 

 ponents of these ranges are of later age, including 

 Palaeozoic formations, chiefly quartzites and lime- 

 stones, granitic intrusions of post-Palaeozoic age, 

 contemporaneous with, or possibly somewhat later 

 than, the batholiths to the north and west, and also 

 stillyounger deformed but unmetamorphosed strata. 



Ilie granitic core of the Sierra Nevada forms the 

 largest of the batholiths mentioned above. The 

 granite masses of the Sierra Madre, San Jacinto and 

 San Bernardino ranges of southern California, are 

 now generally considered as outlving members of 

 the main batholith. Most batholithic masses, when 

 studied in detail, have been found to be complex, 

 consisting of rocks of varying ages, the older mem- 

 bers of which have been caught up and fused into 

 the later intrusives. It is not improbable, there- 

 fore, that the older gneisses and schists appearing 

 within the less metamorphosed granites, are of pre- 

 Cambrian age, but until these are studied in detail, 

 the entire complex is best classified as late Jurassic 

 in age. 



Certain apparently ancient schists of Siskiyou 

 County, California, have been provisionally assigned 

 to the pre-Cambrian, and the somewhat similar 

 Colebrook schists of Oregon may belong here. The 

 basin in which the late pre-Cambrian formations 

 of the Belt series were deposited, lies just east of 

 the boundary line between Washington and Idaho. 

 Certain still older gneisses and schists which form 

 the basement upon which the Belt sediments were 

 laid down occur in northeastern Washington. 



These scattered fragments of the ancient rock 

 formations furnish few data from which we may 

 reconstruct the geography of the west coast region, 



43 



