SKETCH OF 4.GINA AND METHANA. 31 
some of the more decomposed specimens they are colored yellow 
or reddish brown on the edges. No pleochroism is to be 
observed, and the extinction angle is decidedly high, in many 
cases reaching or surpassing 45°. Hour-glass structure is not 
uncommon, though never well developed. Though the crystals 
are asa rule simple, yet twins, generally with the twinning plane 
a (100), are to be noted; and in some cases interpenetration 
twins, of two prisms crossing at angles of about 76° and the clino- 
pinacoids being parallel. These apparently have the twinning 
plane ¢ (oor), and resemble the augite twins described by Elich* 
in andesites from Ecuador. 
The crystals are frequently grouped in Clusters of irregular 
shape, and with no definite arrangement of the component crys- 
tals, somewhat resembling in structure, but not mineral compo- 
sition, the clusters of Judd’s glomero-porphyritic structure.? 
These again sometimes assume the form of rings of augite crys- 
tals (sections of spheroidal shells), which surround a nucleus of 
clear, colorless substance which generally shows perlitic cracks 
and only rarely exhibits a faint double refraction. This is with- 
out doubt glass, and in the few cases where it acts on polarized 
light the effect may be caused by a state of strain due to the 
growth of the augite shell. Projecting into this glass nucleus 
from the inner edge of, and at right angles to, the augite ring, 
are small colorless or greenish prismatic crystals, which present 
all the characteristics of augite. They resemble the augite fringes 
often seen around enclosed quartz grains and may be of secondary 
origin. 
The hornblende of these andesites belongs to the brown 
variety and megascopically much resembles the so-called basaltic 
hornblende, though the color is blacker and the surfaces more 
glistening. 
Microscopically they are seen to be larger than the green 
TREISS and STUBEL. Reisenin Siid America. Das Hochgebirge, I. Rep. Ecuador, 
I. West Cordillers iii., atacatzo bis liniza von E. Elich. Berlin 1893, p. 157, Pl. ii, 
Fig, 4. 
2JupD. Q.J. Geol. Soc. XLII. 1886, p. 71. 
