204 GEOLOGICAL RECONNAISSANCE OF THE DOMINICAN REPUBLIC. 

 THE CORDILLERA CENTRAL. 



The Cordillera Central was examined along the cross mountain trails 

 through Restauracion and Constanza, those being the only feasible routes 

 of travel northward to the Cibao Valley. Only the parts south of Restau- 

 racion and Constanza are described here. Plate XVI, B, shows a view in 

 the Cordillera Central looking southwestward across the frontier of Haiti 

 from a place on the Banica-Restauracion trail near La Cruz, in the Prov- 

 ince of Monte Cristi. 



The high mountains north of San Juan Valley are made up of a large 

 variety of rocks, both igneous and sedimentary, consisting in part of 

 quartz diorite and related crystalline rocks, which have intruded and 

 more of less extensively metamorphosed older crystalline and sedimentary 

 rocks. The foothills are faced with white, finely crystalline limestone, 

 which appears in bold exposures that stand out conspicuously as seen from 

 far out in the valley. Resting on and cutting the metamorphosed sedi- 

 mentary beds and diorite are volcanic rocks of several kinds that for con- 

 venience are designated the "early volcanics." They include dikes, lavas, 

 and volcanoclastics, generally of basic variety, well advanced in alteration 

 to serpentine. There are in addition the "late volcanics," of andesitic to 

 basaltic composition, probably of Pleistocene age. These occur here and 

 there in the valley and also cover small areas in the mountians. They are 

 easily distinguished from the "early volcanics" by their slight decomposi- 

 tion and their lack of deformation. 



The great San Juan Valley is bordered on the north by a conspicuous 

 ridge of gray crystalline fossiliferous limestone, which forms a facing for 

 higher mountains opposite the town of San Juan, and thence extends 

 westward to Banica as a spur from the main mountain mass, 300 to 500 

 meters above the surrounding plain. The limestone is finely crystalline, 

 is greatly seamed by veinlets of calcite, and contains indeterminable 

 Foraminifera. 



Back of the limestone ridge and possibly conformably underlying it is a 

 thick series of beds of hard shaly sandstone and sandy shale, which extend 

 northward for many kilometers and form high mountains near the border 

 of the Republic of Haiti. The hardness of these beds is the result of 

 regional metamorphism that appears to become increasingly pronounced 

 toward the north. The bedding lines in the shale are in large part ob- 

 scured by a slaty cleavage having a general northwest strike. In the 

 exposures farthest north the beds of shale have been changed to phyllite 

 and the few beds of limestone have been changed to marble. From the 

 vicinity of Guayajayuco southward to the mouth of Rio Joca, Rio Guaya- 

 jayuco flows through a sharp canyon cut in this rock, which is a dark 

 phyllite intricately seamed with white veinlets that run at right angles to 

 the schistosity. In passing this canyon the trail along the Dominican 



