514 O. H. HERSHEY GEOLOGICAL RECONNAISSANCE IN NICARAGUA 



Panama mine, on the opposite side of the rhyolite area, a vein in ande- 

 site is unusually rich in galena and other sulphides. Thus I have gained 

 the idea that veins cutting the basic rocks are richer in sulphides and 

 other basic minerals than those in and near the rhyolite area, which I 

 consider an indication that the rhyolite as a large mass will continue to 

 a much greater depth than the present valley floors. 



The Big Falls on the Pis-Pis Eiver is caused by a narrow band of 

 purple gray to nearly white, hard, flinty appearing rock that seems to 

 rise through the andesites like a large dike trending about north 60 

 degrees east (magnetic). Lawson identified it as rhyolite. 



X. The rock has a reddish browu color, banded appearance, and a porphy- 

 ritic structure. The feldspars, which were determined as oligoclase-andesine, 

 occur as phenocrysts, but are rare and are full of secondary sericite. The 

 ground-mass is glassy, with an attempt at crystallization, as it is composed in 

 places of crystallites. In the ground-mass occur lenses of quartz, feldspar, 

 and glass, which are evidently zones of flowage. Magnetite and hematite occur 

 on the edges of these flow zones. The hematite gives the rock the red color. 



DIORITE ( ?) 



The only other rock seen in place in the district is in an area 1,000 

 |eet long and 270 feet in maximum width, forming the foot-wall of a 

 portion of the Bonanza vein. I have not seen it in a fresh condition, but 

 the general appearance of partially decomposed material suggests that it 

 was a diorite. Diorite and ordinary biotite granite occur somewhere on 

 the southeast border of the district, as indicated by gravels of the Tunkey 

 Eiver. 



MISCELLANEOUS \ 



The coarse volcanic agglomerate at AYaspuc Mouth extends up th& 

 Waspuc Eiver several miles. Beyond that it is fine grained and appar-. 

 ently well stratified. At Yahook Falls it contains beds of very hard, 

 dark gray (almost black) obsidian, a thick layer of which causes a ver- 

 tical fall of 8 feet. A gorge 6 miles farther up is in highly altered, 

 rather coarse-textured, massive andesite. Five miles farther up the rock 

 is distinctly bedded, dipping downstream 30 degrees. Thence to the 

 mouth of the Pis-Pis Eiver andesite lava prevails, though at one place 

 there was noted a fine-grained lava of more acid appearance and at 

 another a dark gray, rather fine-grained basic crystalline, suggesting the 

 intrusive andesites. At the Yapooketan Falls the Pis-Pis Eiver is cas- 

 cading over a fine-grained, lavender rock that I think is an andesitic tuff 



