510 O. H. HERSHEY GEOLOGICAL RECONNAISSANCE IN NICARAGUA 



Big Falls, including the vicinity of San Pedro. It forms part of the 

 hanging wall of the Bonanza vein toward the northeast and the foot-wall 

 toward the southwest. The northeastern part of the Mars vein is largely 

 in it. It occurs extensively as wall rock to the veins in the southwestern 

 portion of the district, including the Siempre Viva, El Vesuvio, La 

 Leticia, La Constancia, and Santo Domingo veins. It is the rock that 

 De Kalb called diorite, Connelly andesite, and Carter porphyry. A 

 specimen representing the foot-wall rock at the Siempre Viva mine, but 

 secured from a boulder in the dirt back of the mill, was described as 

 follows : 



III. In thin-section the rock is of a medium-grained porphyritic structure 

 with a glassy ground-mass. The original ferro-magnesian mineral is absent, 

 being replaced by epidote and calcite. Unlilie section No. 1, there is very little 

 chlorite, the decomposition being more in the nature of the separation of mag- 

 netite. This occurs in very fine grains, shot through the glassy ground-mass, 

 giving it a black appearance. The rock itself is extremely magnetic. The 

 feldspars are in amount the most important mineral. These were determined 

 as labradorite. They occur as large phenocrysts and as small lath-shaped 

 crystals imbedded in the glassy ground-mass. These small feldspars show a 

 tendency toward orientation in a general direction, indicating flow structure. 



The extrusive andesites are apparently intruded by harder, heavier, 

 darker crystallines which resist decomposition better and are most likely 

 to outcrop and produce residual boulders. From their tendency to ap- 

 pear near the surface as dark green rocks, I used for them the field 

 designation "greenstone." They decompose near the surface to a stiff 

 bright red clay, but the decomposition does not extend nearly as deep as 

 in the other rocks. At the Panama mine some of the material seems to 

 be in small dikes cutting an andesite agglomerate. 



On the hanging-wall side of the Bonanza vein there is an oval area of 

 "greenstone" 500 feet long and 285 feet in maximum width. It does not 

 quite touch the vein and on that side contacts with the ordinary ande- 

 site. On the west it is bordered by an acid intrusive rock. A hard 

 kernel from the Bonanza mill grade was identified by Lawson as holo- 

 crystalline andesite and described as follows : 



II. The rock has a greenish color, is fine-grained and holo-crystalline. In 

 thin-section it seems to be made up of an intergrowth of augite and feldspar, 

 which are about of the same size. Also augite occurs rather abundantly as 

 large crystals, which are on the average larger than the feldspars. Magnetite 

 is an important inclusion in the augite. Considerable of the augite has been 

 altered to a green fibrous hornblende and to chlorite. The labradorite which 

 makes up most of the rock occurs as tabular crystals, of varying sizes. The 

 rock seems to have a porphyritic structure, as in general there are two sets of 



