202 GEOLOGICAL RECONNAISSANCE OF THE DOMINICAN REPUBLIC. 



QUATERNARY SEDIMENTS. 

 "coast limestone." 

 Along and near the coast in the Provinces of Barahona and Azua are 

 locally flat-lying limestone and calcareous conglomerate called by Gabb 

 the "Coast limestone." In the vicinity of Barahona village the rocks 

 form terraced sea cliffs, and to the south, where the mountains lie near the 

 6ea, they consist largely of firmly cemented limestone conglomerate and 

 reef-coral material. Somewhat similar deposits extend inland along the 

 south border of Enriquillo Basin (PI. XVII, B). (For lists of fossils 

 see pp. 167-168.) 



ALLUVIAL DEPOSITS. 



South of Azua the material penetrated in drilling water wells appears to 

 be in large part unconsolidated sand and gravel. Many deposits of such 

 material are later than the "Coast limestone," including outwash gravels 

 along the base of the mountains and sand and gravel along modern river 

 channels and on terraces bordering the valleys. Part of the delta deposits 

 of Rio Yaque and other streams and of the alluvium that forms the floor 

 of Enriquillo Basin may be of Pleistocene age, but most of it is probably 

 Recent. 



IGNEOUS ROCKS. 



As already stated, large parts of the Cordillera Central consist of crystal- 

 line igneous rocks ranging in composition from quartz diorite to grano- 

 diorite. Others consist of serpentine, probably altered basic igneous rock. 

 The diversity of igneous rocks found in the gravel of almost any large river 

 flowing from the Cordillera Central shows that the basal complex includes 

 a wide variety of types. No adequate petrographic study of these rocks 

 was made. Quartz diorite and granodiorite appear to compose the bulk 

 of the larger batholithic masses. Nearly all the crystalline igneous rocks 

 show evidence of dynamic metamorphism, and many have been changed 

 to gneisses, but in few places has the metamorphism gone far enough to 

 obscure the original character of the rock. 



Besides the coarsely crystalline igneous rocks there is a thick series of 

 volcanic rocks in the Cordillera Central, which are in part lavas and in 

 part fragmental rocks, including tuffs and breccias. These are characteris- 

 tically dark purple to green in color and vary in texture from fine-grained 

 to coarse. They are metamorphosed, chloritized, and in part altered to 

 serpentine. The more basic types of lavas are more common than the 

 silicic types. 



In San Juan Valley and here and there in the mountains of the two 

 provinces there are small areas of volcanic rocks of Pliocene or more recent 

 age. Some of these are fine-grained porphyritic lavas of andesitic compo- 

 sition but there are also basalts. In Sierra Bahoruco near the seacoast 

 there are amygdaloidal basalts. 



