GEOLOGY OF WEST COAST REGION OF UNITED STATES 

 immediately preceding the great invasion of the 

 sea during the succeeding Palaeozoic era. We 

 know in general that the American continent was 

 elevated above the sea, probably far bevond its 



E resent boundaries, and was gradually reduced in 

 jvel. Towards the close of the pre-Cambrian a 

 great trough, east of Washington, was filled with 

 sediments. Early Cambrian times witnessed an ex- 

 tension of the Great Basin Sea of Nevada and ad- 

 joining states into eastern California. There ex- 

 isted at this time, perhaps, an archipelago of small 

 islands in southeastern California, and low land 

 masses in the region now occupied by the Sierra 

 Nevada Mountains. The Pacific Ocean lay to the 

 west of the present coast line, a condition which 

 probably existed on through Palaeozoic, Triassic, 

 and Jurassic times. 



THE PRE-CRETACEOUS METAMORPHICS. During 

 Palaeozoic times the western strands of the Great 

 Basin Sea occupied the eastern portion of Califor- 

 nia, now advancing, now retreating, and finally, 

 towards the close of the era, reaching well into 

 western California, possibly as far west as the 

 present coast line in the vicinity of Monterey, where 

 the extensive Santa Lucia limestone of Palaeozoic 

 age is well developed. The northern Palaeozoic 

 Ocean occupied a considerable portion of Oregon, 

 but its full extent is not known, as much of both 

 Oregon and Washington are covered by the Colum- 

 bian lavas. 



The Palaeozoic strata are largely the metamor- 

 phosed equivalents of shales and sandstones, but 

 in Devonian and Carboniferous times extensive 

 strata of limestone were deposited. The late 

 Palaeozoic sediments are interstratified with large 

 amounts of volcanic material, lavas and tuffs. The 

 total thickness of this series of coastal deposits is 

 at least 10,000 feet in California. 



The Triassic and Jurassic sediments were laid 

 down in a relatively small inland sea or sound, ex- 

 tending over only the western portion of the region 

 occupied by the former Great Basin Sea. These 

 early Mesozoic seas were undoubtedly connected 

 with the Pacific Ocean, but a great island, includ- 

 ing the western portion of California and Oregon, 

 and probably extending west of the present coast 

 line, separated the inland sea from the Pacific 

 Ocean. The maximum thickness of the marine 

 Triassic and Jurassic of northeastern California is 

 not less than 8000 feet. This suggests that there 

 was a nearby land mass, although no mountain 

 ranges were formed as yet. 

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