LITHOLOGY. 207 
in twin relationship. As there are large accumulations of this rock, it is 
one of considerable importance. 
A hornblende sienite much resembling this comes from Columbia. I 
would like to draw the attention of those interested in microscopic min- 
eralogy to this rock, for in its sections a vast number of cavities filled 
with liquid carbonic acid are found, and, moreover, these cavities possess 
associations that render them very instructive. The rock is white in 
color, spotted with black, and macroscopically only orthoclase and horn- 
blende are visible. In thin sections, plagioclase, biotite, quartz, and apa- 
tite are found; and, moreover, calcite is seen to be a constituent of the 
rock, a mineral which, I think, is not often found in sienites. Quartz is 
present only in small amount, occupying little angular corners; but every 
grain of it is filled with cavities which are quite large, all of which con- 
tain liquid carbonic acid: and this circumstance, in connection with the 
presence of calcite, is very interesting. Knowing that this liquefied 
gas had been found in granitic rocks, I had examined a very large 
number of sections with the idea in view of finding it in our rocks; but 
the tests indicated in all cases nothing but water, and I was therefore 
much pleased to find it in one of the last sections I had to examine, and 
in great abundance. Its presence in connection with the calcite may 
indicate that carbonate of lime was a constituent of the material from 
which this rock was made, and that, at the temperature at which recrys- 
tallization took place, a reaction occurred between the lime carbonate 
and the silicates, producing plagioclase and liberating carbonic acid ; but 
the rock was under such pressure that it could not escape, and it was 
consequently imprisoned in the quartz, which was the last mineral to 
solidify, and the quartz, being small in amount, is correspondingly full 
of the cavities containing the fluid gas. The appearance of a section of 
a quartz grain, as it is seen when magnified 350 diameters, is shown in 
Fig. 1 on Pl. 12, These cavities are often arranged in rows, some of 
which are straight and some curved, and many more cavities are irregu- 
larly strown about. In the larger of them a double line is seen on the 
outer edge, and this line cuts off the corners and irregularities of the 
cavities, and indicates the presence of a very small amount of a second 
fluid, which is probably water. The bubbles are quite large in propor- 
tion to the size of the cavities, and one can immediately perceive that 
