GEOLOGY AND GEOLOGIC HISTORY. 65 



MIOCENE SERIES. 

 Yaque Group. 



PREVIOUS STUDIES. 



The Miocene formations of the Cibao Valley have been studied in greater 

 detail and by more investigators who have published the results of their 

 work than any other strata in the Dominican Republic. T. S. Heneken 1 

 described the geology of part of this region in 1853, but his conception of the 

 geology contains many errors. The collections of fossils procured by him 

 were studied by British paleontologists, 2 whose descriptions give the first 

 account of the Miocene fauna of the island. Gabb 3 criticized scathingly 

 the work of Heneken, but his own interpretation of the geology leaves 

 much to be desired. Dr. Carlotta J. Maury 4 and her associates in the 

 expedition to the Yaque Valley in 1916 brought back large collections of 

 fossils, by means of which she discriminated two faunal zones, the Sconsia 

 laevigata zone and the Aphera islacolonis zone, to which she later 5 applied 

 the names Gurabo formation and Cercado formation, respectively. 



SUBDIVISIONS. 



The work of the expedition of 1919 increased the number of formations 

 discriminated in the Miocene of the valley of Rio Yaque del Norte from 

 two to six, of which the lower two are regarded as approximately contempo- 

 raneous. The names applied to these formations are the following : 



Mao clay. 



Mao Adentro limestone. 

 Gurabo formation. 

 Cercado formation. 

 Baitoa formation. 

 Bulla conglomerate. 



For the six formations of Miocene age in the valley of Rio Yaque del 

 Norte and its tributaries the name Yaque group is here proposed. This 

 name is peculiarly appropriate, because the equivalent strata on the south 

 side of the Cordillera Central are best exposed along Rio Yaque del Sur. 



The formations of the Yaque group in the area south of the Cordillera 

 Central have not been named, although five divisions that are probably 

 worthy of formational rank have been discriminated by Condit and Ross. 

 The generalized section of the Yaque group in the vicinity of Quita Coraza 

 is described by Condit and Ross as follows: 



1 Heneken, T. S., On some Tertiary deposits in Santo Domingo, with notes on the fossil shells by J. C . 

 Moore and on the fossil corals by W. Lonsdale: Geol. Soc. London Quart. Jour., vol. 9, pp. 115-134, 1853. 



2 Duncan, P. M., On the fossil corals (Madreporaria) of the West Indian Islands: Geol. Soc. London 

 Quart. Jour., vol. 19, pp. 406-458, Pis. XIII-XVI, 1863; idem, vol. 20, pp. 20-24; 358-374, Pis. II-V, 

 1864; idem, vol. 24, pp. 9-33, Pis. I, II, 1867. Sowerby, G. B., Descriptions of new species of fossil shells 

 found by T. S. Heneken: Geol Soc. London Quart. Jour., vol. 6, pp. 44-53, 1849. 



3 Gabb., W. M., On the topography and geology of Santo Domingo: Am. Philos. Soc. Trans., vol. 15 

 n. s., p. 95, 1873. 



4 Maury, C. J., Santo Domingo type sections and fossils: Bull. Am. Paleont., vol. 5, Nos. 29, 30, 1917 . 

 8 Maury, C. J., Science, new eer., vol. 50, p. 591, 1919. 



