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University of California. 



[Vol. 2 



interior is hard and often horizontally jointed, giving it a quasi- 

 stratiform appearance. This was identified as rhyolite, but the 

 fragment examined was small, and although the rock has the com- 

 position of a soda-rhyolite, its occurrence as an intrusive, and its 

 coarse granitic structure, would seem rather to entitle it to be 

 termed an alkaline granite. 



The bulk of the main range or Sierra Balcazar, at the head of 

 the Santa Maria River, was found to be composed of a very hard 

 dark gray massive rock, which has been doubtfully identified by 

 Dr. Grant as andesite, although upon closer examination, he thinks, 

 it may prove a trachyte or even a diabase. In places it contains 

 large phenocrysts of hornblende. This andesitic rock occurs in 

 the deep valley of the Guaxaro River on the north of the divide; 

 but a few miles farther north, a ridge nearly equally as high as the 

 main divide, separating the Guaxaro from the Bijuco and Saltos 

 Rivers, is composed to the very top of a massive coarse crystalline. 

 This same rock is exposed to low levels in the valley of the Bijuco 

 River. Dr. Grant says it is " composed mostly of flesh-colored feld- 

 spar, with some biotite and perhaps a little hornblende or augite," 

 appearing like some of the nepheline syenites. 



A little farther northeast, the Saltos and Santiago Rivers are 

 flowing over precipices and huge boulders of the fine-grained dark 

 gray and black andesite (?). The relation between the three rock 

 types was not well made out, as the country is a dense jungle, but 

 in general it may be said that the coarse crystallines form the core 

 of the cordillera and the andesitic rock occurs as a thick coat 

 originally completely covering them, but largely . removed by 

 erosion. 



It is probable that the acid crystallines occur in the form of 

 huge batholites which invaded the supposed andesite. But for 

 evidence of their having been intruded subsequent to the late 

 Eocene basic eruptive epoch, they might be supposed to represent 

 the plutonic phase of the same volcanic activity as produced the 

 Panama formation. Doubtless, these Veraguas crystallines (syenite 

 and alkaline granite) belong to the same system of "pseudo granite 

 or syenite" rocks mentioned by Hill as exposed in the plateau of 

 Costa Rica, the Sierra San Bias, around the Sierra del Marta, on 



