CHAPTER IV. 



STRATIGRAPHIC AND STRUCTURAL GEOLOGY AND 

 GEOLOGIC HISTORY. 



By Wythe Cooke. 



STRATIGRAPHY. 



BASAL COMPLEX. 

 CHARACTER AND EXTENT. 



The oldest rocks in the Dominican Republic form a complex group of 

 schists, serpentines, intrusive and extrusive igneous rocks, tuffs, and more 

 or less altered conglomerates, shales, and limestones. These ancient basal 

 rocks resemble the basal rocks in Cuba, Porto Rico, and the Virgin Islands, 

 are genetically related to them, and have suffered similar metamorphism. 

 This basal complex forms the axis of the Cordillera Central and makes up 

 a large part of Samana Peninsula. (See PI. VIII.) 



The complex includes rocks of many kinds, and a part of it certainly 

 dates from Cretaceous time, but part is probably older. Some of the 

 igneous and pyroclastic rocks that are now included in it are of Tertiary age, 

 for they are intruded into or interbedded with Tertiary sediments. To 

 untangle the intricacies of the basal complex, to map its component parts, 

 and to ascertain the relative ages of the various rocks will require long and 

 intensive work in both field and laboratory — work which cannot be done 

 until suitable base maps have been made. 



CORDILLERA CENTRAL. 



Along the Bonao trail from Santo Domingo to La Vega the first outcrop 

 of rocks referable to the basal complex is about 5 kilometers north of Los 

 Alcarrizos, where brecciated augite andesite that weathers into spheroidal 

 masses, outlined by rhombohedral joints, is exposed. No other outcrops 

 were noted on this trail before Hatillo was reached. 



Most of the rock exposed between Hatillo and El Madrigal is schistose 

 serpentine. In the road cuts near Hatillo the serpentine is much sheared 

 and folded, and is metamorphosed into a talcose slate. The rock was 

 probably originally lava and pyroclastic material of intermediate composi- 

 tion. It is cut in places by dikes of fresher and less sheared black rock, 

 which shows a tendency to ophitic texture. 



Several of the hills between Hatillo and El Madrigal are capped with 

 limonitic boulders and pebbles. Near Rio Jaina there are several exposures 

 of creamy-yellow rock, probably weathered sandstone. In the bed of Rio 

 Jaina at the first crossing there are many boulders of dioritic gneiss, but 



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