1915] Clark: Occurrence of OUgocene in the Contra Costa Hills 11 



Two important papers published during the last two years have 

 added considerably to our knowledge of the Tertiary faunas of the 

 North Pacific Coast. The first of these was by Dr. C. E. Weaver. 9 

 The second paper, by Arnold and Hannibal, 10 undertakes a broader 

 and more general correlation. Beds which are placed in the Oligocene 

 are referred to the Astoria series, which is divided into three forma- 

 tions — the San Lorenzo, Seattle, and Twin River. These formational 

 names are apparently not used in the lithologic sense but in the 

 faunal. The lowest of the three faunas is recognized as being equiv- 

 alent to the San Lorenzo of the Santa Cruz Quadrangle, California. 

 Beds which are called the Monterey are said to overlie the Astoria 

 series. An unconformity is inferred between the Monterey and the 

 Astoria series, but no definite description is given. 



Weaver in describing the Oligocene and Miocene horizons did not 

 recognize any of the older and more general names. Beds, the faunas 

 of which are equivalent in part at least to the Astoria series of Arnold 

 and Hannibal, are called by him the Lincoln and Blakeley formations. 

 The former is referred to the Oligocene, the latter to the Miocene. 

 Weaver apparently uses the term "formation" in a lithologic sense. 



In a recent paper, Chester W. Washburne 11 gives a brief description 

 of the sedimentary formations in northwestern Oregon. According 

 to Washburne, no evidence has been obtained in that region which 

 indicates a stratigraphic break between beds regarded as Oligocene 

 and those of upper Eocene age. It is stated also that there is no reason, 

 stratigraphic or lithologic. for the separation of these Oligocene beds 

 from beds which are considered to be of Miocene age. The evidence 

 for making the division was entirely palaeontologic, being based upon 

 the work of Dr. W. H. Dall. It is stated that Arnold and Hannibal 

 do not agree with this division, since they place the lower part of the 

 Miocene as recognized by Dall in the vicinity of Astoria, Oregon, as 

 a part of the Oligocene. 



In the Contra Costa hills the beds, which in this paper are referred 

 to the Oligocene, have heretofore been included in the Miocene and 

 considered to be a part of the Monterey Group. However, the dis- 

 tinctness of this fauna has long been recognized. In 1904 Dr. J. C. 

 Merriam 12 notes that the fauna of the beds referred to the Monterey 



9 Weaver, C. E., A preliminary report on the Tertiary palaeontology of 

 Western Washington, Washington Geol. Surv. Bull., no. 15, pp. 1-80, 1912. 



10 Arnold, Ralph, and Hannibal, Harold, op. cit. 



11 Washburne, Chester W., Eeconnaissance of the geology and oil prospects 

 of Northwestern Oregon, U. S. G. S. Bull., 590, pp. 1-110, 1914. 



12 Merriam, J. G, A note on the fauna of the Lower Miocene in California, 

 Univ. Calif. Publ., Bull. Dept. Geol., vol. 3, no. 16, p. 378, 1904. 



