﻿28 A. Hague and J. P. Iddings — Volcanic Rocks 



the usual inclusions of glass and magnetite. A rhombic 

 pyroxene of light brown color in thin section, with marked 

 pleochroism, green parallel to the c axis and light reddish 

 brown at right angles to it, has the same microscopical char- 

 acter as that found in many andesites in the Great Basin and 

 Pacific coast volcanoes, which chemical analysis has shown to 

 be a variety of hypersthene. It is only found in those basalts 

 poor in olivine. Small crystals and irregular grains of magne- 

 tite are always present. 



The general structure of these basalts is porphyritic, with the 

 porphyritic crystals in greater or less amounts, usually the 

 feldspars are more abundant than the other macroscopic con- 

 stituents, though in a few instances the olivine is the most 

 prominent. Only in one of the basalts is there an approach to 

 granular structure, and this occurs on the summit of the crater 

 San Miguel ; at the same locality also occur several of the most 

 porphyritic and fine-grained varieties. 



Pyroxene-Andesite. — These rocks are represented by fewer 

 specimens than the basalts and in general are not in as fresh a 

 condition. They are dark colored rocks, with many small por- 

 phyritic feldspars, 2 or 3 mm long, and fewer macroscopic pyrox- 

 enes. Eighteen thin sections show that in almost every case 

 there is a very fine-grained, holocrystalline groundmass of 

 feldspar, pyroxene and magnetite grains, with a greater or less 

 amount of porphyritic crystals, of which plagioclase is the most 

 abundant. The pyroxene is only in a few instances entirely 

 fresh. In these cases it is seen to be of two varieties, a light 

 green augite and a pleochroic, pale brown, orthorhombic form, 

 which, as already remarked, corresponds in all its microscop- 

 ical characters to the hypersthene of the Great Basin rocks. 

 The rock from Cerro de Cunchique is a pyroxene-andesite 

 with small amount of olivine; one of the intermediate forms 

 between these andesites and basalt. 



Hornblende- PyroxeneAj i desite. — This is represented in the 

 collection by still fewer specimens, mostly pi>miceous in char- 

 acter. They are glassy, fresh rocks with porphyritic crystals 

 of feldspar, hornblende and pyroxene. In one specimen from 

 -San Sebastian quarry there occur a few scattered grains of 

 crackled quartz, and in rock from the volcano of Ilopango occa- 

 sional grains of olivine. No mica was observed either in the 

 hand specimens or in thin sections. 



The porphyritic feldspars are all plagioclase, apparently in 

 the labradorite-andesine series, with numerous inclusions and 

 other features characteristic of the feldspars of glassy andesue. 

 The hornblende is of a brown color, in some cases with a red- 

 dish, in others with a greenish tinge. It is without black 

 border, and in not perfectly developed crystals. The margins 



