206 GEOLOGICAL RECONNAISSANCE OF THE DOMINICAN REPUBLIC. 



were found in it, its age remains in doubt. Fragments of limestone breccia 

 were found beside the trail a little farther down, and at La Madre Vieja, 

 a spring about 1 kilometer beyond the outcrop of limestone, there were 

 rolled boulders of conglomerate. 



Massive red rock, weathering shaly and inclosing pebble-like lumps, 

 forms the base of Loma la Laguna on its northern side, but higher there are 

 large blocks of massive conglomerate, composed of many varieties of igneous 

 rocks, and less abundant fragments of limestone. At the top of the moun- 

 tain is massive blue limestone conglomerate containing large boulders of 

 gray cherty limestone. 



The bluff of Rio de las Cuevas where the Constanza trail meets it is 

 composed of very coarse conglomerate with calcareous cement. It con- 

 sists chiefly of boulders of igneous rocks, especially gray andesite, but it 

 contains also travel-rounded boulders of limestone conglomerate similar to 

 that at the top of Loma la Laguna. There are evidently conglomerates of 

 two ages in this region and the limestone is older than either. The con- 

 glomerate at the river appears to dip 18° N., but its bedding is obscure. 



RIO YAQUE VALLEY. 



In its course from the high mountains to the sea Rio Yaque del Sur 

 crosses weak Oligocene and Miocene sandstones and shales, which are 

 exposed along the valley in cliffs on alternate sides almost continuously as 

 far down as Alpargatal, where the river emerges from the hills and flows 

 slowly through alluvial lands for the rest of its course. The opportunity 

 for stratigraphic study is so excellent that a week was spent in examining 

 the exposures from the vicinity of Tiibano on Rio de las Cuevas down 

 to Alpargatal. A rough compass traverse was carried the entire distance, 

 and supplemental traverses were run to nearby points of interest along the 

 route. An attempt was made to ascertain the stratigraphic sequence of 

 the beds and to determine the possibility of their being oil-bearing under 

 cover. As a result it is possible to give a section that is believed to be 

 representative of at least the south-central part of the Province of Azua. 

 At several places the river has cut close to the base of mountainous ridges 

 of early Tertiary limestone. It crosses a spur of the Neiba Range just 

 below the mouth of Rio San Juan, and farther down it runs just west of 

 Sierra Martin Garcia. 



A good opportunity to study the character and thickness of the strata 

 where they are but little affected by faulting was found in the vicinity of 

 Quita Coraza, where beds of the Yaque group dip northward from the 

 flank of Sierra Martin Garcia into a synclinal trough. The following 

 section is a compilation of measurements made in that vicinity: 



