62 University of California Publications in Geology [Vol- H 



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Arnold was the first to recognize Oligoeene in California. 27 In 

 his paper on the "Tertiary and Quaternary pectens of California," 

 the San Lorenzo formation is described and referred to the Oligoeene, 

 a fauna of over fifty species being listed from these beds. The fol- 

 lowing extract from this paper gives Arnold's principal reasons for 

 calling these beds Oligoeene : 



The fauna of the San Lorenzo formation consists, for the most part, of forms 

 best suited by conditions prevalent during the deposition of sandy shales. In 

 other words, it is a moderately deep-water fauna. It shows several species found 

 in the Monterey shale, but it also contains many species which appear to be 

 closely related to Tejon Eocene forms. Bearing in mind these faunal relations 

 and the stratigraphic position of the formation, it appears probable that it 

 belongs in the Oligoeene. 



The new species from the San Lorenzo formation were described by 

 Arnold in a paper entitled "Descriptions of New Cretaceous and Ter- 

 tiary fossils from the Santa Cruz Mountains, California." 28 Later, 

 a more complete description of these beds, together with a map show- 

 ing their areal distribution, was given in the Santa Cruz Folio of 

 the United States Geological Survey. 29 The San Lorenzo of the type 

 section has a maximum thickness of twenty-five hundred feet, being 

 composed chiefly of fine-grained sandstones and shale. The final 

 conclusion of those who studied this section was that this formation 

 graded up into the Lower Miocene (Vaqueros). Certain beds be- 

 tween tlie two were designated as transitional Oligoeene-Miocene. 



During the last ten years considerable work has been done in 

 mapping the geology of the Coast Ranges of California, and with 

 this has come a much better knowledge of the stratigraphy and faunal 

 sequence than was had before. The United States Geological Survey 

 has been one of the chief contributors to this work. A number of 

 bulletins pertaining to the geology and oil resources of California 

 have appeared from time to time, and these have been a great stimulus 

 to the students of geology and palaeontology on the West Coast. The 

 statement is made in several of these bulletins that certain of the 

 beds described may be of Oligoeene age. In their report on the 

 McKittrick-Sunset oil region, Arnold and Johnson 30 describe certain 

 beds which they think may possibly belong to that period ; in the 



27 Arnold, Ralph, U. 8. Geol. Surv., Prof, paper no. 47, pp. 15-17, 1906. 



28 Arnold, Ralph, Proe. U. S. Nat. Mus., vol. 34. no. 1617, pp. 345-390, 1908. 



29 Brainier, J. C, Newsom, J. F., and Arnold, Ralph, Santa Cruz Folio 163, 

 U. S. Geol. Surv., p. 3, 1909. 



so Arnold, Ralph, and Johnson, Harry, A preliminary report on the McKittriek 

 Sunset oil region, Kern and San Luis Obispo counties, California, U. S. Geol. 

 Surv., Bull. 406, p. 41, 1910. 



