Art, 12, SIDERITE AND ASSOCIATED MINERALS—SHANNON. 5 
scope the fresh gray rock of the light variety is seen to be a normal 
basalt consisting of twinned laths of plagioclase in a mesh of perfectly 
fresh augite, the texture being typically ophitic, although some of 
the pyroxene grains have a tendency to euhedral development. 
Large euhedra of iron ore are scattered throughout the section, and 
there occurs interstitially a moderate amount of brownish glass filled 
with minute acicular microlites. 
The sections of the dark rock show only one essential difference 
from those of the light rock. This dark rock is composed of the 
same perfectly fresh assemblage of plagioclase, which by its extinc- 
tion angles is shown to be bytownite, with augite and accessory iron 
ore and glass. Throughout the rock, however, there are interstitial 
areas of a transparent yellow-brown isotropic material having an 
index of refraction of 1.432 to 1.438, which is evidently opal colored 
by hydrated ferric oxide. The opal occupies the same relative posi- 
tion as the glass with relation to the other constituents, and it may 
represent an alteration product or replacement of the glass. That this 
material is not merely glass stained by iron is shown by its much 
lower index of refraction. The microlites which are present in the 
glass do not occur in the opal. The other constituents of the rock 
are free from any sign of alteration or staining, and areas of glass 
which still remain in the sections which contain the opal show no 
incipient or partial replacement by the opal. 
THE VESICULAR CAVITIES. 
The cavities are varied in size from minute to some which are 
15 cm. or more in diameter. They are most abundant in the more 
glassy forms of the basalt, but the larger ones are in the dense crys- 
talline rocks. Many of these vesicles are nearly spherical and repre- 
sent typical gas bubbles, while others are much flattened and deformed 
as though by movement after the enclosing lava had become very 
viscid. It is with the several minerals which occur in these cavities 
that this paper has chiefly to do. These are described in some detail 
below, even though the majority of them are not particularly rare or 
remarkable species. The minerals are, with one or two exceptions, 
discussed essentially in the order of their deposition. 
PLAGIOCLASE (OLIGOCLASE-ANDESINE). 
The first lining of the vesicles of the freshest rock is composed 
either of a grayish glassy enamel-like coating or of a crust of dis- 
tinctly lath-like crystals of plagioclase. The largest of these feldspar 
crystals reach 2 mm. in length. Upon microscopic examination the 
gray glaze is found, like the crystals, to consist of feldspar, there 
being no isotropic material present in the enamel. Every gradation 
exists between the gray enamel and the crystal crusts, intermediate 
