512 O. H. HERSHEY GEOLOGICAL RECONNAISSANCE IN NICARAGUA 



IV. In thin-section tlie rocli lias a greenish color, is fine-grained, and has a 

 slight porphyritic structure, with a glassy gro«nd-mass in which the small 

 crystals are imbedded. The augite occurs as well developed crystals and is 

 in an unaltered condition, except that on the edges occurs a rim of chlorite 

 and magnet'ite. The feldspars are the most abundant mineral and are basic- 

 oligoclase. They occur as large phenocrysts, and as small lath-shaped feld- 

 spars imbedded in the glassy ground-mass. 



Along the Lone Star ditch there seems to be a rapid alternation of the 

 purple-weathering andesite and the harder, dark greenish gray rock, the 

 latter probably occurring as dikes. The Lone Star Falls is due to a 

 small area of hard, fine-grained, greenish gray rock like that at Bonanza 

 Falls. Thence by the trail to the old Santo Domingo arrastre only the 

 extrusive andesite was seen, but many hard boulders of "greenstone" 

 occur near the arrastre. Thence to the Constancia mine the andesite 

 lava probably contains dikes of the harder rock. The hanging wall of 

 the Siempre Viva vein as exposed in the "N'ew" tunnel is a very hard, 

 fine-grained, nearly black rock that has been identified by Lawson as 

 augite andesite and described as follows : 



VI. The rock is dark-colored and fine-grained. In thin-section it shows a 

 porphyritiQ structure, with phenocrysts of augite and plagioclase feldspar. 

 The ground-mass is glassy, in which occur lath-shaped feldspars, magnetite, 

 and chlorite. The augite occurs as large phenocrysts and as small flakes in 

 the ground-mass ; it has been nearly all altered to chlorite and magnetite. 

 Green fibrous hornblende occurs as an alteration replacement product of the 

 augite, in long prisms in the ground-mass. 



The Siempre Viva Falls is in two parts, one 90 feet high and the other 

 75 feet high, with rapids below; the entire fall is 220 feet. They are 

 caused by a rib of hard, massive, greenish gray, porphyritic rock, resem- 

 bling that at the Bonanza Falls except that many of the white feldspar 

 phenocrysts have a tendency to a lath shape, giving the rock a diabasic 

 appearance. 



RHYOLITE 



The northeastern portion of the district is characterized by a mass of 

 acid intrusive rock about 4 miles long in a northeasterly direction and a 

 mile in maximum width. It forms a lighter red and less claye}^ soil than 

 do the basic rocks, but it is weathered to so great depth that I only found 

 fresh material at two places. Both have been identified by Lawson as 

 rhyolite. Specimen VIII is from the face of the Banana tunnel at the 

 Bonanza mine and specimen IX from a boulder in a creek near the 

 Hidden Treasure mine. 



