Clark: The San Lorenzo Series of Middle California 67 



Miocene of California. An unconformity is inferred to exist between 

 the latter beds and those of the Astoria series. 



Besides the three Oligocene formations referred to above, the San 

 Lorenzo, Seattle and Twin River, Arnold and Hannibal placed the 

 Sooke beds of Vancouver Island in the Oligocene. These beds have 

 already been referred to as having been described by Merriam (see 

 p. 59), and considered by him to be Upper Miocene in age. Only 

 a brief statement is given by Arnold and Hannibal concerning the 

 fauna and stratigraphy of the Sooke beds. Apparently, however, 

 there was no doubt in their minds but that the Sooke fauna came 

 from beds which were stratigraphically lower than their San Lorenzo 

 formation as recognized in Oregon and Washington. 



Reference has already been made to the paper by the writer 43 in 

 which it was first announced that certain of the beds heretofore in- 

 cluded in the Miocene were in reality Oligocene in age. The present 

 paper is a more complete review of the evidence for this separation, 

 together with a description of the fauna. 



A recent paper by Chester Washburne, 44 "Reconnaissance of the 

 geology and oil prospects of northwestern Oregon," is of especial 

 interest in connection with the problem of the relationship of the 

 Oligocene and Miocene as recognized in Oregon, in that here we find 

 the latest views of W. H. Dall as to the point at which the separation 

 should come. Washburn gives a detailed description of the lithology 

 and occurrence of the formations in the vicinity of Astoria, Oregon. 

 Eocene, Oligocene and Miocene beds are recognized in that section. 

 It is stated that tuffaceous Eocene beds are overlain, probably uncon- 

 formably, by the Astoria shale, the thickness of which is estimated 

 to be about fourteen hundred feet. 



The Astoria shale, so named by Thomas Condon, is stratigraphically an indi- 

 visible lithologic unit, being a homogeneous mass of dark-colored marine shale. 

 The lower 400 feet or more is of Oligocene age and the remaining 1000 feet or 

 less is Miocene, probably Lower Miocene. In the field there is no way of dis- 

 tinguishing the two parts of the formation. So many fossil species pass across 

 the invisible dividing line that a collection made below the top of the Aturia 

 zone (Oligocene) at Knappton, Washington, opposite Astoria, has been referred 

 to the Miocene. Aturia angustata, which is regarded as typical of the Oligocene, 

 has been found in concretions from 300 to 400 feet above the base of the forma- 

 tion. Dall later used the expression Astoria group for both the Miocene shale 

 and overlying sandstone, excluding the Oligocene part of the shale. The name 



43 Clark, B. L., The occurrence of Oligocene in the Contra Costa Hills of Middle 

 California, Univ. Calif. Publ., Bull. Dept. Geol., vol. 9, no. 2, pp. 9-21, 1915. 

 "Washburne, C. W., IT. S. Geol. Surv., Bull. 590, pp. 1-108, 1914. 

 45 Washburne, C, op. cit., pp. 15-16'. 



