LITHOLOGY. 20/ 



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 siHcates, 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. I 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 



