1917] Nomland: The Etchegoin Pliocene of Middle California 225 



Martin 41 the lower Pliocene beds found near Sargent, called San Pablo 

 by Jones, are referred to the same horizon as the Etchegoin. Martin 

 also states that a portion of the Purisima is probably equivalent to a 

 portion of the Etchegoin of Arnold and Anderson. 



AGE DETERMINATION 



As will be seen by a comparison of the faunal lists, the lower 

 Etchegoin has a greater percentage of Recent species than the upper 

 division or than the formation as a whole. According to the usual 

 custom of calculating by the Lyell percentage method, the lower 

 Etchegoin should therefore be placed later in the geologic time-scale 

 than the upper Etchegoin. In this instance we know by undoubted 

 stratigraphic evidence that this is not a fact. We know also that 

 during the deposition of approximately 7000 feet of Pliocene a con- 

 siderable faunal evolution must have occurred. 



That the percentage method has only a very limited degree of 

 accuracy was shown by Dr. W. H. Dall 42 nearly twenty-five years ago 

 in the following statements : 



There is no doubt that Lyell 's hypothesis has been of great use in settling 

 early Neozoic nomenclature, and has generally hitherto been applied in a manner 

 to which little exception could be taken. But the old conception of the mathe- 

 matical individuality of species has passed away, never to return, and the 

 numerical estimates based upon it are no longer practicable in the absence of 

 any method of determining the personal equation of different palaeontologists 

 in their estimates of what constitutes a species. 



The classification retaining these names is no longer numerical, but strati- 

 graphic and developmental, and the formations classified under a given name 

 are, for the writer at least, not necessarily synchronous, except where strati- 

 graphically continuous, or synchronic only in a very wide and general sense. 



It is believed that American geologists are well agreed that the minor sub- 

 divisions of the systems cannot in America at present be subjected to any rigid 

 parallelism with the minor subdivisions of other lands, and that the difficulty of 

 correlation increases with the differences of latitude and distance. Concurring 

 in this opinion, theoretically and practically, no attempt at such correlation has 

 been attempted by the writer within the geological limits assigned to him. 



It appears, therefore, that the Lyell percentage method can be 

 applied only to the larger divisions and only in a very general way. 

 The very fine distinctions which have frequently been made as to the 

 relative age of many of the California Tertiary formations based on 



« Martin, Bruce, The Pliocene of Middle and Northern California, Univ. 

 Calif. Publ. Bull. Dept. Geol., vol. 9, no. 15, 1916. 



« Dall, W. H., Correlation papers, Neocene, U. S. Geol. Surv. Bull. 84, p. 179, 

 1892. 



