30 THE JOURNAL OF GEOLOGY. 
being seen in any of the specimens, The groundmass is almost 
without exception dark gray, in some of the Mt. Chondos speci- 
mens almost black, quite compact and occasionally with a sub-— 
greasy luster. The rock near the tuff at the west end of Mt. 
Chondos shows a banded structure of alternate dark and light gray 
streaks,which run almost vertically ; they are best brought out on 
slightly weathered surfaces. 
The only exception to the above general description is the 
hornblende-augite-andesite specimen from a small hill or ridge 
southeast of Kakoperato, which is much less compact in texture, 
lighter in the color of the groundmass, and shows a great many 
phenocrysts of hornblende. 
Allthese andesites show a great tendency toward decompo- 
sition, becoming dull, more or less friable, and of a light reddish 
or purple color. Endogenous enclosures are quite common in 
the rocks of this group and will be described later. 
Under the microscope the groundmass of these rocks is seen 
to be not typically hyalopilitic, but consisting of a glass base 
which is either colorless or brown according to the megascopic 
appearance of the rock, containing numerous microlites of plagi- 
oclase and augite with many magnetite grains and very much 
fine globulitic “dust.” In the groundmass are also seen many 
larger plagioclase lathes, with small, stout, colorless augite crys- 
tals, and fewer small hornblende crystals, many of which are 
quite fresh and of a yellow-brown color, while others are altered 
to a dark opacitic mass either on the edges or completely. Here 
again while the distinction between the groundmass and pheno- 
crystic plagioclase is easy on account of their diverse habits, as 
in the Stavro rocks, in the case of the augite and hornblende the 
line of demarkation cannot be sharply drawn, and the distinction 
is almost entirely a question of size, though the difference in the 
amount of alteration in the small and large- hornblende crystals 
is of some assistance. 
Augite occurs in not very large, usually well shaped crystals, 
showing the common planes of a clear and extremely pale 
brownish yellow color, so faint as to be almost colorless. In 
