GEOLOGY OF THE PROVINCES OF BARAHONA AND AZUA. 209 



pose the hills to the north. Near the mouth of Rio San Juan the beds are 

 inclined northward at an angle of 50° and overlie the shale and limestone 

 of Eocene and Oligocene age that form the massif of the Neiba Range. 

 (For lists of fossils see p. 109, stations 8564, 8565, 8619; pp. 157-162, sta- 

 tions 8563 and 8566.) From this point northward along the Yaque the dip 

 at first is northward at a low angle, but it gradually steepens to more 

 than 60° near the mouth of Rio de las Cuevas. Some of the beds contain 

 plentiful plant fossils, and others contain fragments of shells and branching 

 corals, which are most abundant in the conglomeratic sandstone layers. 

 The younger deposits are of Oligocene and Miocene age. 



The Miocene strata near the mouth of Rio San Juan appear to rest on a 

 hard, coral-bearing limestone, about 30 meters thick, which contains 

 poorly preserved Foraminifera that are questionably referred to the Eocene. 

 This limestone forms the summit of a high hill on the east side of the road 

 about 1 kilometer south of La Trinchera. Beneath it are at least 1,000 

 meters of more or less sandy bluish shale containing calcareous layers 

 that abound with a species of Nummulites of Eocene age. The bluff on the 

 left bank of the river below La Trinchera consists of this shale, which is 

 characterized by many thin sandy laminae, each less than 1 centimeter 

 thick, spaced at intervals of 3 to 4 centimeters through the shale. At the 

 first bluff on the right bank a few kilometers below La Trinchera this 

 shale lies against Eocene limestone in possible fault contact. 



The hills on the east side of Rio Yaque opposite the mouth of Rio San 

 Juan (see PI. XVIII, B) are capped with basaltic lava that may have come 

 from a volcanic fissure in the high hills a short distance to the south, oppo- 

 site La Trinchera, where basalt cuts the Eocene and Miocene country rock. 



The gravel of the Las Matas formation, which occupies so large a part 

 of the great San Juan Valley west of Sierra del Agua, may extend in a con- 

 tinuous belt around the north end of those hills to Rio Yaque, where the 

 gravels form an extensive lowland at the confluence of the Yaque with its 

 tributaries Rio del Medio and Rio de las Cuevas. 



About 6 kilometers farther east up Rio de las Cuevas there are jagged 

 hills of sandstone and conglomerate of the Yaque group. The rocks 

 at this place dip steeply southwestward and apparently rest unconformably 

 on a conglomerate composed largely of fine-textured gray limestone, which 

 resembles a platy, thin-bedded, nonfossiliferous limestone that outcrops 

 along the valley a little farther upstream, near Tiibano. 



SAN JUAN VALLEY. 

 General Features. 



The Gran Valle de San Juan, with its extensive savannas and low hills, is 

 made up largely of loosely consolidated gravel of the Las Matas formation. 

 Interbedded with the gravel are a little sandstone, a little marly limestone, 



