TERTIARY ROCKS 509 



mentals were distinctly seen at three places and it is probable that they 

 make up a large part of the series. The fresh rock is generally of a dark 

 greenish gray color and fairly hard, but at most of the outcrops the 

 andesite lava has a dark red to purple color because of partial decompo- 

 sition. Weathering converts some of the rock to a soft, deep-red mass 

 with white spots due to the kaolinized feldspars. This is the form in 

 which it is generally seen in the walls of the veins. Near the surface it 

 becomes a mass of stiff red clay. 



A specimen from the trail on the north of the Mars vein has been 

 identified by Lawson as andesite and described as follows : 



I. Ill thin-section the rock shows a medium-grained structure, with feldspar 

 crystals of porphj-ritic habit, in a fine-grained and glassy ground-mass. The 

 rock shows signs of severe alteration, as the feldspars are full of calcite and 

 kaolin, and the ground-mass is full of calcite, chlorite, and epidote. The rock 

 is essentially composed of plagioclase feldspars of two generations ; these make 

 up the bulk of the section. The original ferro-magnesian minerals are entirely 

 lacking, having been replaced by the epidote, calcite, and chlorite, which occur, 

 especially the chlorite in porphyritic-shaped masses. Magnetite occurs through- 

 out the section, but due to alteration ; most of it has been altered to limonite. 



Along the Mars power-line, one-half of a mile northwest of the Mars 

 mine, there are hard residual boulders that have weathered out of the 

 reddish-stained andesite, and a specimen has been described, under the 

 term "augite andesite," as follows : 



VII. The rock is dark colored, medium-grained, with a decidedly porphyritic 

 structure, the feldspars occurring as large phenocrysts in a dark glassy ground- 

 mass. Fine grains of magnetite give the glass the dark color. The alignment 

 of the phenocrysts gives evidence of a flow structure. Augite occurs as well 

 developed phenocrj^sts. There is a slight attempt at a diallage cleavage in 

 cross-sections of the augite prisms. The augite occurs in crystals up to one- 

 eighth of an inch long. A large amount of the augite has undergone altera- 

 tion, and has been replaced by chlorite, epidote, and magnetite. The augite 

 has been formed prior to the feldspars. Magnetite occurs abundantly as large 

 crystals of primary origin, and as small secondary magnetite, as an alteration 

 product of the augite. The feldspars are the most abundant mineral present. 

 They are perfectly fresli and unaltered. Their extinction angle shows them to 

 be labradorite. They occur as large phenocrysts one-quarter of an inch long 

 and as small lath-shaped crystals in the ground-mass. 



This rock I believe to be fairly typical of the majority of andesite out- 

 crops seen in ascending the Wanks, Waspuc, and Pis-Pis rivers, and it 

 evidently has a wide distribution in northeastern Nicaragua, particiilarly 

 in the valleys and under the old coastal plain. There seems to be a 

 large, scarcely interrupted area of it along the Pis-Pis River above the 



