140 JOURNAL OF THE [june 



Branchville, Mr. Wright found about seven-hundredths of one 

 per cent, of liquid. In the quartz of the Cornish granytes the 

 cavities sometimes make up at least five per cent, of its volume, 

 and the water in them, on an average, about one per cent, of its 

 volume, or four-tenths of one per cent, of its w^eight. As the 

 quartz-grains in a granyte or in gneiss rarely amount to less 

 than Miy per cent, of the v^hole rock, it is probable that the 

 liquid contents of such a granyte commonly reach as much as 

 one-fifth of one per cent.; i. e., about four pounds of liquid 

 to a ton of the rock. 



The contents of these cavities are, in general, air or nitrogen 

 or some other gas, such as carbon dioxide ; but nitrogen seems 

 to predominate. In studying a section, one will often be 

 surprised to observe the wide variation in the character of the 

 different cavities in close approximation. Some are completely 

 filled with liquid, some half filled, some empty, — that is, occu- 

 pied merely by gas. The most common inclusion in the cavities 

 consists of a liquid, usually transparent and colorless, which may 

 be either water or brine. By brine I mean a solution more or 

 less strong of some salt, often supersaturated. One or more 

 crystals of the salt are often seen adhering to the walls. On 

 gently heating a thin section containing an inclusion of this 

 kind, the crystal sometimes disappears, and, on cooling, it re- 

 appears, occasionally in the form of several crystals. These 

 crystals consist mostly of potassium chloride, sodium chloride, 

 and calcium sulphate, or gypsum. They are generally attached 

 firmly to the sides, or wedged in some narrow corner. Some 

 have been found loose, so that, on revolving the section, the 

 crystal was seen to tumble around the cavity ; but this occur- 

 rence is very rare. It is of interest to note, that in not a single 

 case has the " Brownian Movement " been observed to affect 

 the crystals or other solid particles occurring in a fluid-cavity. 



The occurrence of carbon dioxide in these cavities has been 

 already mentioned. Its liquid form has a specific gravity of 

 only 0.6, water being taken as unity. This may be easily recog- 

 nized," when it nearly fills a cavity, by the peculiar limpidness 

 of the liquid and its freedom from color, by the delicacy of the 

 outlines of the gas-bubble floating within it, and by the phenom- 



**See the excellent papers by W. N. Hartley, Jour. Chem. Soc, London, 1876, pp. 

 137-43 ; and 1877, pp. 237-50. 



