GEOLOGY OF THE PROVINCES OF BARAHONA AND AZUA. 201 



For lack of time it was not practicable to map separately the Eocene and 

 Oligocene deposits and they were therefore considered together, but in 

 subsequent work the rocks belonging to the two series should be discrimi- 

 nated. The fossils most useful in the determination of the age of the early 

 Tertiary beds are the Foraminifera, which appear to be equally abundant 

 in the limestones, the sandy beds, and even the coarse conglomerates, but 

 they are so inconspicuous that they may readily be overlooked. 



Miocene. 



YAQTTE GROUP. 



The rocks of Miocene age, besides occupying the greater part of the 

 Cibao Valley, also occupy the principal structural basins in the Provinces 

 of Azua and Barahona. The strata exposed along Rio Yaque del Sur 

 are at least 1,500 meters thick. Sandy conglomeratic strata predominate 

 but there are also shales and limestones. The deposits are almost entirely 

 marine but include local beds of lignite and fossil plants, which denote 

 temporary emergence. These beds belong to the Yaque group as defined 

 by Cooke. (See pp. 65-66, and for lists of fossils see pp. 156-162.) 



CERROS DE SAL FORMATION. 



Around the border of Enriquillo Basin there are Miocene strata which 

 are obviously much younger than those found along the upper course of 

 Rio Yaque. They comprise the gypsum and salt-bearing beds, the fossils 

 of which indicate late Miocene age. To these strata the name Cerros de 

 Sal is applied, from the locality of that name, near which rock salt is obtained. 

 Until a more detailed study is made it will not be possible to define 

 sharply the boundary between these beds and the underlying Miocene 

 strata. As stated above, the lowest strata of the Miocene found along the 

 upper course of Rio Yaque are largely sandstone. It is not known whether 

 the Yaque group occurs in the Bahoruco Mountains. A great thickness 

 of impure limestone, probably of Miocene age, underlies the Cerros de Sal 

 formation, but its lithologic character is so different from that of the Yaque 

 formation that there is considerable uncertainty as to their relations. 

 (For lists of fossils, see pp. 163-164.) 



Pliocene. 



LAS MATAS FORMATION. 



There is no positive evidence of the presence of Pliocene deposits in the 

 Dominican Republic. In San Juan Valley and to the southeast, along and 

 near Rio Yaque, the plicated Miocene and older strata are overlapped by 

 thick deposits of loosely cemented gravels and clays which generally dip 

 gently basinward or are folded to a much less extent than underlying 

 strata. These beds are called the Las Matas formation, for they are 

 typically exposed near the town of Las Matas. No fossils have been 

 observed in these gravels, but it is surmised that they are of Pliocene age. 



