GEOLOGY AND GEOLOGIC HISTORY. 51 



the bedrock is green chloritic schist. The profile of some of the mountains 

 far west of this point suggests that they are composed of thick-bedded 

 sediments that dip gently southward. 



At the second crossing of Rio Jaina, about 12 kilometers upstream from 

 El Madrigal, an outcrop of weathered tuff or similar rock was seen. The 

 gravel in the river here consists of diorite, a crystalline dark igneous rock, 

 and a smaller amount of more siliceous intrusive rock that contains dark 

 inclusions. No pebbles of sedimentary rocks were seen. Arroyo Gua- 

 nanitos at its junction with Rio Jaina flows over hard, tough, and some- 

 what schistose greenish amphibolite, which weathers to a soft, rusty 

 brown rock above river level. Near Sabana Grande the bedrock is gray 

 dioritic gneiss containing much biotite. 



Between Sabana Grande and Bonao all the rocks are metamorphic. 

 They are mostly micaceous, chloritic, sericitic, and quartzose schists and 

 various metamorphosed intrusives. Between Rio Juan Manuel and Rio 

 Maimon a dark, schistose amphibolite is exposed. The cobbles in Rio 

 Maimon are principally coarse and slightly gneissoid diorite, but the bed- 

 rock is a greenish slate or phyllite, in places cut and altered by dikes. 



Between Bonao and Jayaco no outcrops of hard rock were noted. The 

 alluvial plains in this region are composed of clay and loam interbedded 

 with gravel, which should be a valuable source of well-water. The gravel 

 in the bed of Rio Jima consists largely of porphyritic andesite but includes 

 some banded slate and basalt. Near the last ford on Rio Jima pebbles of 

 lignite were noted, which were probably derived from a low bluff near by. 



Loma Miranda, the last mountain crossed on this trail, is composed of 

 sericite schist and schistose serpentine cut by dikes of andesite or basalt, 

 which show little or no schistosity. Outcrops of brilliant red gossan and 

 quartz veins containing numerous leached cavities show the effect of miner- 

 alizing solutions, which probably were associated with the intrusion of the 

 dikes mentioned above. Some prospecting for copper has been carried on 

 in this vicinity, but so far without marked success. Cuts along the new 

 Duarte Highway will afford good fresh exposures of rocks of the basal com- 

 plex. In the bed of Rio Jima near kilometer 96 from Santo Domingo on 

 this road there are boulders of conglomerate that contain pebbles distorted 

 by squeezing. 



Along the road from La Vega to Cevicos, on a low ridge 2 kilometers 

 west of the crossing of Rio Yuna, fragments of white tuff were noted, and 

 2 kilometers east of the river there is a higher ridge composed of fine-grained 

 granitic rock associated with tuffs. On the low round hills known as the 

 Cerros de la Travesia, west of Rio Chacuey, there are exposures of light 

 gray or yellowish tuff. The first ridge east of Rio Chacuey contains 

 greenish-gray volcanic rocks. Loma de los Palos is composed of basalt. 

 The tuffs and effusives along the northern foothills of the Cordillera 



