453 Dominican Sections — Maury 37 



Olivella cf. indivisa Gpy. Scapharcariocanensis?, n. sp. (young) 



Marginella cercadensis, n. sp. Phacoides yaquensis Gabb. 



Strom binacf. pseudohaitensis, n. sp. Phacoides cercadica, n. sp. 



Natica sulcata Born. Pitaria circinata Born. 



Polinices subclausa Sby. Pitaria cf. acuticostata Gabb. 



Melanella cf. cercadica, n. sp. Tellina scitula Dall. 



Cadulus phenax P. and S. Siliqua subsequalis Gabb. 

 Corbula cercadica, n. sp. 



Of the 19 species in this list 15 are present in Bluff 3. The 

 reciprocal percentage is 7. Undoubtedly this horizon goes with 

 Bluff 3 at Cercado on the Mao. 



Discussion of the Stratigraphy 



Moore raised the question whether the collections made by 

 Heneken in the Yaqui Valley might not represent more than one 

 formation, and in late years Dall has suggested the same query. 

 Heneken and Gabb, however, both thought that the fossils were 

 from one formation. Gabb* states very emphatically that the 

 whole valley is made up entirely and only of late Miocene strata 

 and that no vertical differentiation of the horizons is possible. 



To determine whether this was the case, or whether more 

 than one formation was present, and to find the stratigraphic 

 succession has been our main object. 



While collecting from the bluffs of the Gurabo river on our 

 1916 Expedition, Mr. Olsson and Mr. Schmidt were struck by 

 the change of fauna in Zone G from that of Zones A-F. A 

 similar change in the faunas was found between Bluffs 2 and 3 

 and Bluff 1 on the Rio Mao. They were convinced that two 

 formations were represented. 



Subsequent critical study of all the various types of life we 

 collected from the bluffs of the Gurabo, Mao and Cana Rivers has 

 shown that their conclusion was justified. 



The Geographical Sequence. — As the prevailing dip is a gen- 

 tle one to the north, on going southward in ascending the south- 

 ern tributaries of the Yaqui, as the Mao, Gurabo and Cana, one 



-Trans. Amer. Phil. Soc, vol. 15, p. 101 and p. 158, 1873. 



