I9I3.] STRATIGRAPHY OF PACIFIC COAST OF AMERICA. 579 



Twin River formation (zone of AcUa gcttysburgensis 

 Rgn., TnrriteUa orcgonensis Conr., and PoUnices 

 olympidii Rgn.). 

 Seattle formation (zone of Acila gcttysburgensis Rgn., 

 Astoria Turciciila icashingtoniana Dall, TnrriteUa nezvcomhei 



Series 1 Mrm., and Macrocallista vcspertina Conr.). 



San Lorenzo formation (zone of Acila shumardi Dall and 

 dalli Arn., TnrriteUa neit'combei Mrm., Tiircicnla 

 colnmbiana Dall, and MacrocaUista pittsbnrgensis 

 Dall). 



The average thickness of the Astoria Series is not less than 

 12,000 feet, but at some points it attains a much greater develop- 

 ment. In the Cape Flattery section about 17,000 feet of apparently 

 conformable coarse sandstones and conglomerates, derived largely 

 from the bedrock series of Vancouver Island, from their fossil con- 

 tents appear to belong exclusively to the San Lorenzo horizon. The 

 base of the section is cut off by faulting at the mouth of the Soo-es, 

 River while the uppermost beds pitch beneath the waters of the 

 Straits of Fuca. Between Winlock and Shoalwater Bay, also in, 

 Washington, is a monotonous westward dipping succession of the 

 Astoria Series which if aggregated would total more than 50,000- 

 feet of beds. The paucity of outcrops and the recurrence of certain 

 igneous flows and tuffs associated with the same basal San Lorenzo 

 fauna suggests the presence of a repetition by step faulting which, 

 with the limited time spent in this district of heavy forests, it was 

 impractical to trace out. 



The San Lorenso Formation. 

 The name San Lorenzo formation has been used by the senior 

 writer^^ for a series of sandstones and diatomaceous shales in the 

 Santa Cruz Mountains, California. Nearly the entire San Lorenzo 

 fauna reappears at a definite horizon in the Tertiary of the North 

 Pacific Coast, i. e., the lowest faunal division of the Astoria Series. 



28 Prof. Pap. 47, U. S. Geol. Sur., 1906, p. 16; Santa Cruz Folio No. 163, 

 U. S. Geol. Sur., 1909; Proc. U. S. Nat. Mus., XXXIV., No. 1617, 1908, 

 P- 348. 



