208 GEOLOGICAL RECONNAISSANCE OF THE DOMINICAN REPUBLIC. 



the bluish-gray to olive-green clay that constitutes the greater part of the 

 section. Both the clay and limestone contain a large number of fossils, 

 including corals of several species (stations 8590; for lists of fossils see 

 pp. 157-162). The clay-limestone beds form a prominent hogback bluff 

 along the right bank of the river opposite Quita Coraza and extend south- 

 westward for many kilometers to the lowlands near Hatico. 



This part of the section is of chief interest as a possible container of oil. 

 The thin beds of coarse sandstone in the midst of the clay-limestone mem- 

 ber might serve as the reservoir rock and the richly f ossiliferous beds might 

 have furnished petroliferous material. The overlying clays would serve 

 as a cap rock, preventing the escape of the oil into the higher sandy beds. 



Above the calcareous beds is coarse sandstone that includes some con- 

 glomerate and some shaly and calcareous beds in its lower part. The sandy 

 beds contain large Areas, and some of the calcareous beds contain many 

 corals. These strata appear on both sides of the east-west synclinal axis 

 that lies between Los Giiiros and Quita Coraza and are exposed on the 

 south to a thickness of about 175 meters. Higher strata in this sequence 

 are concealed by a covering of gravels of the Las Matas formation, which 

 occupy the axis of the syncline. 



The unconformable relations of the gravels of the Las Matas formation 

 with the underlying strata is not pronounced here, the gravels agreeing 

 rather closely in attitude with the substrata. In fact they would be taken 

 for the same formation by one who had not become familiar with the litho- 

 logic character of the two and had not seen the decidedly unconformable 

 relations at places up the river. The gravels of the Las Matas formation 

 here, as usual, are less firmly consolidated than the deposits composing 

 the Yaque group, and they contain no marine fossils. Interbedded with 

 the gravel, which is yellowish gray, are layers of soft, marly limestone and 

 layers of faint reddish to purplish clay, which differs strikingly in color 

 from the bluish-gray to olive-green clays of the Yaque group. The un- 

 conformity between the two formations is plainly shown at a locality 

 3 kilometers upstream from Los Giiiros. Here the gravel of the Las Matas 

 formation forms a cliff on the left bank of the river, whereas a short dis- 

 tance farther upstream the water flows in a cataract over fossiliferous 

 sandstone of the Yaque group. The unconformity between the two 

 formations is very marked, for there is discordance in the strikes as well as 

 the dips. 



The general character of the strata along the upper course of Rio Yaque 

 in the vicinity of the crossing of the Azua-San Juan road is described below. 

 (See PI. XVIII, A.) From the mouth of Ilio de Las Cuevas down to the 

 mouth of Rio San Juan there are thick deposits of sandstone and conglom- 

 erate, which are considered the basal part of the Yaque group. The same 

 strata appear for a distance of several kilometers up Rio San Juan and com- 



