228 PROCEEDINGS OF THE PALEONTOLOGICAL SOCIETY 



Oligoeeiie iu middle California." ^ Briefly stated, the conclusions were as fol- 

 lows : At the base of the Monterey group, i;n certain localities to the west of 

 Mount Diablo in the Concord quadrangle, a|^ described and mapped In the San 

 Francisco Folio,- a portion of the lower sandstones, designated as the Sobrante 

 formation, contains a very distinct fauna from that found in the sandstones 

 and shales immediately above ; the paleontologic and stratigraphic evidence 

 indicates that these lower beds belong to a distinct period of deposition, there 

 being a marked stratigraphic, as well as a faunal, break between the two. 

 The name Agasoma gravidum zone was applied to the lower beds; the fauna 

 in the beds immediately above the break was referred to the Area montcreyana 

 zone. The fauna of the Agasoma gravidum zone was correlated with that of 

 certain beds in Washington and Oregon, which beds have been referred to the 

 Oligocene by different writers. 



Since the publication of this paper more extensive field-work has been done 

 and the wi'iter has been able to get much better acquainted with the Oligocene 

 fauna of Oregon, Washington, and Vancouver Island. The term Astoria series, 

 as used here, was first applied by Arnold and Hannibal as a general name for 

 the Oligocene of the west coast in their paper entitled "The marine Tertiary 

 stratigraphy of the North Pacific coast." ^ This name is synonymous with the 

 term Clallam formation, as used by Prof. C. E. Weaver in his recent paper en- 

 titled "Tertiary faunal horizons of western Washington." * 



Beds in the region of Mount Diablo, referable to the Astoria series, are 

 found in two general sections — one to the west and one to the north of the 

 mountain. These two sections are very different, both lithologically and 

 faunally. The beds to the west of the mountain in the Concord quadrangle 

 have a maximum thickness of onlj'^ a little more than 500 feet. To the west 

 and south of the Concord quadrangle they disappear and the beds of the Area 

 montcreyana zone (Monterey) rest directly on the Tejon (Upper Eocene) or 

 older formations. The Oligocene formations north of the mountain (Mount 

 Diablo quadrangle) have a maximum thickness of over 3,500 feet. They are 

 more heterogeneous than those found in the Concord quadrangle and contain 

 at least one disconf ormity ; thej^ are overlain unconformably by the San Pablo 

 group, the Monterey group being absent. The fauna obtained from these beds 

 on the north side of the mountain and on which the determination of their 

 age is based came from near the top of the series. At the present time some- 

 thing like forty marine invertebrate species have been found in the Oligocene 

 beds to the north of Mount Diablo, while over one hundred species have been 

 obtained from the beds to the west of the mountain. 



The fauna obtained from the Astoria series in the region of Mount Diablo, 

 considered as a whole, appears to be more closely related to the Lower Oligo- 

 cene of Washington and Oregon than to the Upper, a number of identical or 

 very closely related forms being common to these beds and the beds in Wash- 

 ington assigned by Weaver to the Lower Oligocene, the "Lincoln horizon" 

 (Molopophorus lincolnensis zone), which in part is the equivalent of the San 



1 B. L. Clarke : Univ. Calif. Publ., Dept. Geol., vol. 9, no. 2, 1915, pp. 19-21. 



2 A. C. Lawson : U. S. Geol. Surv. Atlas, San Francisco Polio (no. 193), 1914. 



3 Ralph Arnold and Harold Hannibal : Proc. Amer. Phil. Soc, vol. Ill, 1913, pp. 576- 

 585. 



* C. E. Weaver : Univ. Wash. Publ. iu Geol., vol. i, no. 1, 191G, p. 4. 



