530 Arnold Heim — 



America, in 1917. They seem to correspond closely with the 

 stratigraphic divisions of the authors' preliminary notes. However, 

 according to letters from Dr. Arnold and Professor Clark, some 

 objections have been made by a United States geologist, who 

 recently has visited the region of La Purisima. He considers the 

 Pnrisima Nueva Formation as identical with the Isidro Formation, 

 and suggests the elimination of the name of Purisima Nueva 

 Formation. This delicate question will be discussed under the 

 preliminary classification, which is maintained with reservation. 



Most of the balance of the fossil collection is formed of casts 

 only, which do not render possible a specific determination. The 

 genera are given according to Professor Clark, to whom the writer 

 is much indebted. 



Previous Investigations. 



The previous knowledge of the stratigraphy of Southern Lower 

 California was fully cited and discussed in 1912 by Bailey Willis,^ 

 according to the " Geologic Map of North America " by B. Willis 

 and G. W. Stose. The most important question discussed by 

 Willis is the determination of the age of the so-called Mesa Sandstone 

 (Mesa = table). 



Gabb ^ considered the Mesa Sandstone to be probably of Miocene 

 age, while the later authors, Lindgren, Emmons, and Merrill, 

 regarded it as Cretaceous. Willis followed these latter authors, 

 according to the following quotation from his book : " The Mesa 

 sandstones of Lower California, which were described by Gabb on 

 very slight evidence as possibly Miocene, are here assigned to the 

 Upper Cretaceous on the evidence of their similarity to the sandstone 

 of that age which occurs at Ensenada." Thus, with the exception 

 of some Quaternary along the coast, the entire peninsula from 

 Todos Santos and La Paz in the south to Punta Abrojos 

 (26° 45' Lat. N.) was considered to be of Upper Cretaceous age. 



Indeed, after having been ashore at Santa Catarina Landing on 

 the west coast (Lat. 29° 31-|-') the author was able to. confirm the 

 Upper Cretaceous age of the " Mesa Sandstone " of that northern 

 part of the peninsula by finding fragments of big Inoceramus already 

 reported by Merrill.^ AU the " Mesa Sandstones " of the southern 

 part of the peninsida, however, as far as the writer was able to 

 determine, are exclusivehj of Tertiary age. As in California, U.S.A., 

 the Tertiary beds of Lower California are divisible into a number 

 of difierent formations or groups separated by well-defined 

 unconformities. The name of "' Mesa Sandstone ", therefore, can 

 no longer be applied to these beds. 



^ B. Willis, Index U.S. Geol. Survey, Prof, paper 71, pp. 644-8. 



^ W. M. Gabb, Report of J. Ross Browne on the Mineral Resources of the States 

 and Territories west of the Rocky Mountains, 1868, pp. 630-9. 



3 George P. Merrill, " Notes on the Geol. and Nat. History of the Peninsula 

 of Lower California," Report of the U.S. National Museum, 1895, p. 976. 



