474 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



] G° C. lower than that at which the quartz of the granite itself cry- 

 stallized; hut, as I shall show helow, it is far more probable that the 

 pressure was very great, and the temperature a dull red heat visible 

 in the dark, and, if so, substituting in (9) the value, ^=43,100, 

 deduced from equation (10), given further on, I calculate by equa- 

 tion (9) that the difference in the temperature must have been about 

 13°. These results clearly show that a great variation in the actual 

 temperature and pressure produces only a small variation in the cal- 

 culated difference in temperature. Perhaps some may think such cal- 

 culations an impossible refinement ; but the facts appear sufficiently 

 distinct to warrant them. In a similar manner I find that the quartz 

 first deposited in a vein in granite at Camborne indicates a tempe- 

 rature quite equal to that at which the granite crystallized ; but in the 

 quartz deposited towards the close of the process the relative size of 

 the vacuities is so much less that the temperature must have fallen 

 fully 30° or 40° C, which is quite probable. 



The number of the fluid-cavities in the quartz of veins is often 

 very great, as if it had been deposited rapidly. They are frequently 

 on an average less than y-^^th of an inch apart, which corresponds 

 to upwards of a thousand millions in a cubic inch ; and they are the 

 chief cause of the very usual whiteness of the mineral. As an illus- 

 tration of their forms, I refer to figs. 49, 50, and 51. Fig. 49 is of 

 very irregular shape, whilst fig. 50 is a tube extremely well fitted for 

 determining the relative size of the vacuity with great accuracy. 

 Clear and transparent crystals contain few or none, as if deposited far 

 more slowly ; and very often crystals, which at their base are white 

 and opaque on account of the number of cavities, are clear and trans- 

 parent at their extremities from containing very few, as though, like 

 what so very commonly happens in making artificial crystals, depo- 

 sition proceeded rapidly at first, but much more slowly towards the 

 close of the process. The form and arrangement of the fluid-cavities 

 are also in every respect analogous to those in crystals prepared arti- 

 ficially ; and every peculiarity in the structure of the quartz of 

 veins, and their relation to the granite, can be most completely ex- 

 plained by supposing that it was deposited from water holding vari- 

 ous salts and acids in solution, at a temperature varying from about 

 200° C. to a dull red heat visible in the dark. In those cases where 

 we must suppose a very high temperature and a great pressure in 

 order to explain the relation between the fluid-cavities in the quartz 

 of the veins and in that of the granite itself, such other minerals as 

 mica, felspar, and tin-ore were often deposited, especially towards the 

 commencement of the process, as if water at a very high temperature 

 were the effective cause of their production. Tin-ore contains many 

 excellent fluid- cavities, though they are usually very small *. 



Of course these conclusions do not apply to all quartz, for, as I 

 have shown, some must have been deposited from nearly pure water ; 



* These deductions are strongly confirmed by the fact, that several of the above- 

 named minerals have been formed artificially by the action of water at tempera- 

 tures similar to those just described. — Senarmont, Ann. de Chimie, 3 e ser. t. xxxii. 

 p. 129 j Daubree, Ann. des Mines, 5 e ser. t. xii. p. 289.— Oct. 1858. 



