438 University of California Publications in Geology [Vol. 8 



This was determined on the basis of the great number of Recent species 

 in the fauna obtained from those beds. Using the same method he 

 placed beds which, in part, correspond to the Monterey Group, in the 

 Miocene. 



C. E. Weaver, 43 in his paper entitled "Stratigraphy and Palaeon- 

 tology of the San Pablo Formation in Middle California," placed the 

 San Pablo in the Pliocene, because of the great number of Recent 

 species which the fauna contained. 



Using in part this same method, Dr. Ralph Arnold in his paper on 

 the Palaeontology of the Coalinga District, determined the Etchegoin, 

 which is now known to be younger than the San Pablo, to be of Mio- 

 cene Age. The following is quoted from page 46 of Arnold's 44 paper: 



According to the list given in this paper, the Etchegoin is represented by 

 84 recognizable species; of these, 55 species or varieties (65%) are extinct, 

 while 27 species (35%) are still living in the Pacific Ocean. According to 

 Lyell 's classification this would place the formation decidedly in the Miocene. 



It is now generally recognized by palaeontologists that the per- 

 centages used by Lyell in applying the "percentage method" must be 

 modified to meet the modern refinement of species. Such a modifica- 

 tion has already been attempted in the Neocene of Europe and in the 

 Neocene of the east coast of North America. 



W. H. Dall, 45 in his paper "The Relation of the Miocene of Europe 

 to that of other Regions and to the Recent Fauna," states that in the 

 molluscan fauna of the Maryland Miocene only 10 per cent of the 

 species survive. He says : 



This small number is partly the result of the rather restricted limits 

 adopted for species by the authors of this part of the volume as compared 

 with the views prevalent in the time of Lyell. However, about 13% of the 

 New Jersey species survive and 14% of the Floridan Chesapeake, so the esti- 

 mate is not far from the normal for the Lower American Miocene. For the 

 Upper Miocene of Duplin about 20% are estimated to survive and 19% in 

 the Suffolk district of Virginia. The intermediate Yorktown beds have about 

 17% of survivors. 



In this paper Dall shows that the percentages of Recent species from 

 the different horizons of the east coast Miocene agree fairly well with 

 those of the corresponding horizons of the European Miocene and 

 apparently the correlation of these different horizons of the Miocene 



*3 Univ. Calif. Publ. Bull., Dept. Geol., vol. 5, no. 16, pp. 268-269. 

 « Bull. U. S. G. S., no. 396, 1909. 



*-> W. H. Dall, Maryland Geol. Survey. Miocene Text, pp. 49-50, 1904. 



