54 University of California Publications in Geology [Vol. ll 



During the last three or four years considerable exploration work 

 upon the Tertiary of Oregon, Washington and Vancouver Island has 

 been done, and large collections from the different horizons have been 

 made. These data have given us a better idea as to the distinct- 

 ness of the different faunal horizons. Probably the most important 

 information obtained is that pertaining to the distinctness of the 

 Oligocene and Miocene faunas in Oregon and "Washington. 



Between 1906 and 1914 very little new information concerning 

 the presence of Oligocene in California was added to that already 

 known. In the summer of 1914, the writer found evidence of a 

 stratigraphic break between beds containing a Lower Miocene fauna 

 and those containing a fauna believed by him to be closely related 

 to that of the San Lorenzo and to the fauna of the Oligocene as 

 recognized in Oregon and Washington. The announcement of this 

 discover was published in a short paper entitled "The occurrence 

 of Oligocene in the Contra Costa Hills of Middle California." 3 Since 

 the publication of this paper, a more careful study has been made 

 of all the sections in the Concord and Mount Diablo quadrangles, 

 where beds containing this fauna are found ; the number of species 

 from these beds has been greatly increased and some important data 

 obtained as to correlation and local stratigraphic relationships. It 

 is the purpose of this paper to give as clearly as possible first, the 

 detailed information concerning the history of the development of 

 our knowledge of the marine Oligocene of the West Coast ; second, 

 a detailed description of the stratigraphy, lithology and fauna of 

 the beds referred to the Oligocene in the region of Mount Diablo ; 

 and third, a discussion of correlation on the West Coast. 



Up to the present time no species have been recognized as common 

 to the East Coast Oligocene and to the fauna referred to the 

 Oligocene on the West Coast. Not only are these faunas specifically 

 different, but they also have a considerably different generic assem- 

 blage. As far as the writer is aware the chief evidence of the 

 Oligocene age of these western beds is their stratigraphic position and 

 relationships, together with the intermediate character of the fauna, 

 taken as a whole, in comparison with that of the Tejon (the upper- 

 most Eocene recognized on the West Coast) and that of the Monterey 

 or Vaqueros (Lower Miocene of the West Coast). The lower beds 

 of this so-called Oligocene contain a number of fairly highly orna- 

 mented molluscan species, which are also found in the fauna of the 

 Tejon. These two faunas also contain a rather large number of 



3 Clark, B. L., Univ. Calif. PubL, Bull. Dept. Geol., vol. 9, pp. 9-21, 1915. 



