212 GEOLOGICAL RECONNAISSANCE OF THE DOMINICAN REPUBLIC. 



From the town of San Juan eastward beyond Rio Mijo gravels of the 

 Las Matas formation are the only rocks encountered. At the crossing 

 of the river is a gravel cliff with a bed of yellowish-gray marl, the strata 

 dipping gently southward. A few kilometers east of Rio Mijo the road 

 enters the hills known as Sierra del Agua, which form the east limit of the 

 Gran Valle de San Juan. The rocks of the Yaque group in the Sierra del 

 Agua are largely bluish-gray sandstone (weathering to olive green), inter- 

 bedded with conglomerate. They contain many fragments of corals, 

 plants, and other fossils. The beds dip steeply in various directions. Sim- 

 ilar deposits may be seen in continuous exposures along Rio Yaque del Sur. 



AREA EAST OF RIO YAQUE DEL SUR. 



No attempt was made to cover systematically the region from Rio Yaque 

 del Sur eastward to Azua. It was crossed by three different routes — one 

 from Tubano southward through Las Yayas, another along the San Juan 

 road, and another along the trail from Quita Coraza to Azua. 



Most of the area east of Rio Tabara is a gravelly plain covered with a 

 dense growth of mesquite, and bed rock is exposed only in the few hills 

 that project above the plain. West of the Tabara there are fairly plentiful 

 outcrops of gravel of the Las Matas formation, which unconformably 

 overlies the folded beds of the Yaque group. Farther north, where the 

 surface is more hilly, there are extensive areas of the Yaque group and 

 volcanic rock and a small area of early Tertiary limestone. 



The trail from Tubano southward through Las Yayas leads first over 

 a series of steeply tilted and probably faulted calcareous shales inter- 

 bedded with limestone (strike N. 80° E.) and next over hard, brittle yellow- 

 ish limestone of upper Eocene age. Rock of Oligocene age is exposed on 

 Rio de las Cuevas below Tubano. In the vicinity of Higuero Largo frag- 

 ments of white coraliferous limestone (station 8608, mixed Eocene and 

 Miocene; for lists see pp. 106, 157-162), probably much younger than the 

 distorted limestone that crops out in Arroyo Salado nearby, were found. 

 The trail then crosses a series of massive, steeply tilted conglomerate. 



About 3 miles south of Higuero Largo, in a branch of Arroyo Salado, 

 a collection of corals (station 8610; for list see pp. 157-158), including 

 Stylophora sp. and other branching forms, was made from a vertical bed of 

 conglomerate striking north and south and interbedded with shale. This 

 formation is of Miocene age. Non-fossiliferous conglomerate with the 

 same strike but dipping eastward is exposed at Las Charcas. 



Several dikes of hard, dark trap that cut Eocene rocks were seen in the 

 vicinity of Tubano. The contact of the trap with limestone is well exposed 

 in a small hillock known as La Cerrita, on the northern edge of Tubano. 

 Other exposures of the intrusive rock were noted in the Cerro de las 

 Chivas, northeast of the village, and in the upper course of Arroyo Lima. 



