GEOLOGY OF THE PROVINCES OF BARAHONA AND AZUA. 203 



At a locality called Punta Cana, between Las Matas and San Juan, there 

 is a hill composed of lava that appears to be younger than those above men- 

 tioned and that may be of Pleistocene age. It is a fine-grained, fresh- 

 appearing limburgite or basalt with phenocrysts of augite. It varies 

 from dense and massive to amygdaloidal texture. Limburgite or basalt 

 that is regarded as of similar age occurs in nearly flat beds capping mesas 

 and upland areas of gravel elsewhere in San Juan Valley. 



GEOLOGIC STRUCTURE. 



The rugged central axis of Santo Domingo consists largely of igneous and 

 metamorphic rocks, probably the oldest in the island. These are over- 

 lapped on the north and south sides by sediments of Tertiary age. Con- 

 sidered as a whole the island is therefore structurally a great anticline 

 in which there are many flexures and faults. The principal mountain 

 ranges and intermontane valleys that parallel the axis of the island are 

 either up-faulted or down-faulted blocks or are the results of folding, or 

 they have been formed by a combination of faulting and folding. 



Block faulting is the principal factor in the development of broad valleys 

 such as Enriquillo and Cibao. The Cibao, which is flanked on the north by 

 the Cordillera Septentrional, serves as the best known illustration, the 

 broad plain being made up of Miocene strata having a general northward 

 dip, which abut against the upf aulted Eocene and older strata that form the 

 mountain mass to the north. The same type of faulting has produced 

 Enriquillo Basin and probably also San Juan Valley. Similar faulting on 

 a minor scale is evident throughout the Provinces of Barahona and Azua 

 and may be seen along almost any north-south valley, where repetition 

 of beds through faulting forms successive hogback ridges. 



No attempt is made to show these faults on the geologic map, for though 

 many are known, none of them was traced throughout its extent. The 

 principal faults are mentioned in the following description of the areal 

 geology. 



AREAL GEOLOGY. 

 GENERAL FEATURES. 



The surface of the Provinces of Azua and Barahona, as already ex- 

 plained, falls naturally into four principal areas: (1) The Cordillera Central, 

 consisting largely of metamorphic and igneous rocks; (2) the extensive 

 inland valleys, made up of weak, easily eroded rocks but chiefly of struc- 

 tural rather than erosional origin; (3) the low mountain ranges, mostly of 

 semicrystalline limestone, alternating with the valleys; (4) the coastal 

 plain, made up of flat-lying, comparatively young beds of limestone and 

 other rock that overlap the older or folded strata in the coastal region and 

 are represented by gravel, sand, or limestone in the topographic depressions 

 of the interior. The geology of parts of the Provinces of Azua and Bara- 

 hona is shown on Plate XV (p. 186.) 



