TERTIAKY FoKMATIONS. 25 



PLIOCENE. 



Two more or less distinct faunal zones are recognizable in the Pliocene; 

 the lower characterized liv many forms identical to or closely related to those of 

 the Miocene; while the upper appears to laclv these older species and contains 

 but comparatively few forms which are not found living at tiie present time. 

 The fauna of the Purisima formation is characteristic of the lower horizon, 

 while that of the Merced represents the upper. 



/' I 'RISTM. 1 FOR MA TION. 

 TYPE LOCALITY. 



The tj'pc' localit}' is in the vicinity of the mouth of Purisima Creek, San 

 Mateo County, Cal. 



NAME AM) DESCRII'TION. 



This formation was named and descriljcd by H. L. Haehl and Kalph Arnold 

 in Proceedings of the American Philosophical Society, vol. 43, No. 175. l',»()4, pp. 

 22-24. 



DEFINITION. 



The Purisima formation consists of a series of conglomerates, tine sandstones, 

 and sandy shales, with a total thickness of about 800 feet, and is typically 

 develoiH'd in tlie vicinity of the lower portion of Purisima Creek, San Mateo 

 County. riie formation also covers a considera))le territory in other parts of 

 the Santa Cruz quadrangle, especially that portion of the area adjacent to the 

 coast and north of Pescadero Creek. Beds of age probably (^([uivalent to at least 

 a part of the Purisima are known from the Cholame Valle\', INIonterey County, 

 and elsewhere south of the type locality. At least a portion of the beds on Eel 

 River, Humboldt County, are doubtless the equivalent of tlie Purisima. As 

 pointed out by Ashley," the Purisima formation (his "Transition beds'") and its 

 probable equivalents appear in their lower portions to be more or less transitional 

 toward the Miocene. 



Much uncertainty exists in tlie mind of the writer as to the exact relation 

 existing between the Purisima and San Pablo formations. After a careful examina- 

 tion of a large amount of material from the type localities of the two formations 

 and also from supposedly eijuivalent formations in the Salinas Valley and adjacent 

 regions, supplemented by field studies at most of tlie localities fi'om which the 

 material was obtained, it appears evident that the two formations are quite 

 intimately related. Just exactly what the nature of this relation is, however, has 

 not yet been determined. It is a complex problem, and is one which will require 

 much careful field and lalioratory study for its solution. The Pyrisima fauna, taken 

 as a whole, appears to be younger than the aggregate San Palilo fauna, and for 

 the sake of convenience will be tentatively considered as the later of the two. 



n Ashley, i;. H., Proc. Califoriiia Aciid. Sci.. ser. 2, vol. 5, 1895, p. 330. 



