100 GEOLOGICAL RECONNAISSANCE OF THE DOMINICAN REPUBLIC. 



Deposits of nearly the same age as the Gurabo formation are known in 

 Costa Rica, Panama, Colombia, and other parts of northern South America, 

 and probably in Trinidad and Martinique. 



An attempt to correlate the Cercado and Gurabo formations with the 

 Miocene deposits of the southeastern United States involves difficulties 

 because of differences in faunal facies due to differences in climate. The 

 Alum Bluff formation of Florida, comprising, in ascending order, the 

 Chipola marl, Oak Grove sand, and Shoal River marl members, is con- 

 sidered Burdigalian in age. Of these three stratigraphic units the Chi- 

 pola marl was deposited under climatic conditions that most closely ap- 

 proached the conditions in the West Indies, although the fauna of the Chi- 

 pola marl is very warm temperate to subtropical and not tropical. The 

 faunas of the Oak Grove sand and Shoal River marl have a cooler water 

 facies. The Baitoa formation of the Dominican Republic appears to be 

 the correlative of the Chipola marl. In Florida the only subtropical 

 fauna that is younger than the Chipola is the Pliocene Caloosahatchee 

 fauna. In many respects the molluscan faunas of the Cercado and Gurabo 

 formations, as well as that of the Bowden marl, are more similar to the 

 Caloosahatchee fauna than to the Chipola or other Alum Bluff faunas. In 

 1917 Woodring 1 stated that the Bowden marl is distinctly younger than 

 the Alum Bluff formation, and as the Alum Bluff formation was then 

 considered upper Oligocene he suggested that the Bowden marl was Burdi- 

 galian, but the Gurabo formation, like the Bowden marl, is doubtless 

 younger than the Alum Bluff formation, and is probably the equivalent of 

 part of the Chesapeake group, so that in European terminology it is of 

 Helvetian age. 



The correlation suggested in the foregoing paragraph differs from that 

 of Vaughan published in 1918 and 1919 2 in that the Gurabo formation and 

 its correlatives are now regarded as one stage younger and the lower three 

 formations of the Chesapeake group as one stage older, or as the equivalent 

 of the European Helvetian. This opinion is a return to that of Dall 3 as 

 against that of Berry "that the Calvert flora indicates a Tortonian age is as 

 conclusive as intercontinental correlations ever can be." 4 The table 

 on page 57 is adjusted to these modifications in correlation, but as the 

 correlations are still tentative readjustments may yet be made. 



Mao Adentro Limestone. 



Only the corals and mollusks of the Mao Adentro limestone have been 

 identified, but the collections include a few calcareous algae and a con- 

 siderable number of Foraminifera. The corals represent a reef-coral 



1 Woodring, W. P., Johns Hopkins University Circular, March, 1917, p. 254, 1917. 



» U. S. Nat. Mus. Bull. 103, pp. 514-516, 586-593, tables opposite p. 569, 595, 1919. 



1 Dall, W. H., The relations of the Miocene of Maryland to that of other regions and to the Recent 

 fauna: Maryland Geol. Survey, Miocene, p. cxliii, 1904. 



* Berry, E. W., The physical conditions indicated by the flora of the Calvert formation: U. S. Geol. 

 Survey Prof. Paper 98, p. 66, 1916. 



