GEOLOGY OF THE PROVINCES OF BARAHONA AND AZUA. 213 



Loma la Guia, at Las Yayas, is composed of gray andesite containing black 

 acicular crystals of hornblende. A steep hill less than a mile beyond 

 La Guia, showing steeply tilted shale and limestone at the base, appears to 

 be capped by lava, but the summit of the hill was not examined. Along 

 the Azua-San Juan road at several places south of Las Yayas there are 

 strata of the Yaque group, which dip steeply northward and are overlain 

 by the flat-lying coarse gravel of the Las Matas formation, which lies in a 

 porous gray matrix ("caliche"). 



ENRIQUILLO BASIN. 

 Outline of Geology. 



Between Rio Yaque and Lake Enriquillo there is a westward-sloping 

 plain that is interrupted by the low hogback hills on the north side of 

 Laguna Rincon. The strata exposed in the north side of the basin along 

 Rio Yaque represent the lower division of the sandstones and shales of the 

 Yaque group. The beds dip northward into a syncline, along the axis of 

 which there are strata that are probably equivalent to those in the Cerros 

 de Sal, on the south side of the basin (Cerros de Sal formation). The lime- 

 stone mountains of the Neiba Range are visible in the distance to the north, 

 and about 30 kilometers to the west the same mountain front lies close to the 

 shore of Lake Enriquillo. The south side of the basin is bounded by the 

 Cerros de Sal, which contain thick beds of gypsum and rock salt. These 

 hills are locally separated from the Bahoruco Mountains, to the south, 

 by a lowland. Similar gypseous strata appear in the ridges on the north 

 side of Lake Rincon. Lake Enriquillo is bordered by a terrace of corallif- 

 erous limestone having an altitude of about 35 meters above sea level. 

 This is prominently developed at Neiba and Duverge and was recognized 

 as a flat-lying bedded limestone at several places to the east. It is either 

 equivalent to or somewhat younger than the conglomeratic "coast lime- 

 stone" that extends from Cabral southeastward beyond Barahona. 



The higher mountains south of Cerros de Sal consist largely of limestone 

 but include some sandy and shaly strata. The evidence of the fossils col- 

 lected in the front range is not conclusive but indicates that these beds 

 are probably Miocene. In the vicinity of Rancho Viejo, south of Duverge, 

 these beds rest on limestone containing many Foraminifera of Eocene age 

 (station 8626, 8627, list on p. 106 ). In the same region there are areas 

 of volcanic rock, which probably cuts the Eocene limestone. No Oligocene 

 strata were recognized in this vicinity. 



Enriquillo Basin, like other longitudinal valleys that traverse the island, 

 is probably the result of downfolding and blockfaulting, the north side of 

 each valley being downfaulted and in contact with a raised block of harder, 

 older rocks that give a mountainous topography. In general the valley 

 strata dip northward and are more or less repeated by step faulting. 



