The Tertiary of Southern Lower California. 537 



of the Bowden or associated horizon, and straits connecting the 

 Atlantic and Pacific at this stage of the Tertiary is thus conclusively 

 proven. Dr. Hall has compared the Bowden fauna to that of 

 Bordeaux and that of the Aquitanian." ..." Dr. Hall holds that 

 the age of the Bowden beds is late Oligocene.' Dickerson himself, 

 however, considers the Bowden fauna as Miocene, on account of 

 the difference of the mammalia of North and South America during 

 Miocene time : " Thus, diastrophism indicates a Miocene age for 

 the Bowden fauna." 



The fossils mentioned above later were sent by Professor Clark 

 to Dr. T. W. Vaughan, of the U.S. Geological Survey, who with 

 Dr. C. W. Cooke worked over the fauna. The following statement 

 by Dr. Vaughan is taken from a letter to Professor Clark : — 



" The collection is a very interesting one, and it appears both to 

 Cooke and me that it is the equivalent of at least a part of the 

 Gatun Formation of Panama. However, I should like to be 

 somewhat reserved in expressing this opinion until a larger collection 

 is available. 



" As there is a promise of being able to tie the Pacific Coast 

 Tertiary formations of California to those in Central America and 

 the West Indies through the exposures in Lower California, it is 

 highly desirable to have more extensive investigations made in the 

 area in which the specimens were found." 



Dr. Vaughan's and Dr. Cooke's idea agrees with the correlation 

 made by R. Arnold and B. L. Clark in that the typical Apalachicola 

 fauna of Florida is believed to belong to the same general horizon 

 as that of the Gatun beds. 



The lithological facies of the " Purisima nueva Formation " is 

 strictly neritic, partly coastal, as indicated by the frequence of 

 Balanus, and totally different from the Vaquero sandstone of the 

 uppermost Oligocene (Aquitanian) and the following Miocene beds 

 of Upper California. 



Regarding the question of classification, the unnamed U.S. 

 geologist is in contradiction to the writer's preliminary notes : 

 " I found that the formation outcropping in these ' windows ' west 

 of the Mission of La Purisima is not below the diatomaceous shale, 

 but above it, and is the same thing as the Isidro Formation, and 

 the fossils and the lithology are the same." 



The writer fully agrees that in no place the " Purisima nueva 

 Formation " is visible below the Monterey. However, the following 

 observations seemed to be against the younger age of the " Purisima 

 nueva ". 



L The fossil beds at 3'8 kilometres west of the Mission are 

 dipping as much as 70°, i.e. steeper than the Isidro in this region, 

 which forms but gentle folds (PI. XXII). 



2. The Isidro Formation at this locality is overlying the marble- 

 like beds with a strong unconformity (PL XXII). ^ 



^ See these white fossihferous banks in the river-bed, and the unconformity 

 of the Isidro on photograph Fig. 3 of the author's paper, loo. cit., 1916. 



