A. R. Hunt — Vein-Quarts, and Sands. 215 



Groups of water inclusions with proportionate amounts of water 

 and vacuity may indicate a temperature over 342° C. ; or unchanged 

 conditions of heat and pressure during the crystallization of the 

 including mineral, under 342° C. Pure water would suggest the 

 first, while the presence of brine or dissolved salts would'prove the 

 second. A paper read by me at the Belfast Meeting of the British 

 Association was founded on the idea of the heating of a district in 

 the presence of water, with the result of forming hydrous minerals, 

 and fluid inclusions in the altered rocks. But this was only a variant 

 of a forgotten warning of Dr. Sorby twelve years before, viz. : " There 

 is a point which should not be overlooked (I forget whether I alluded 

 to it in my paper), and this is, that a rock formed under one set of 

 conditions may have been exposed to others, and thus the state of 

 the cavities may indicate the change produced by a reheating of 

 rock." This seems to lead to a curious possibility, viz., that a crystal 

 of quartz might catch up fluid with various salts in solution, such as 

 soda, potash, and magnesia, and that on a strong reheating of the 

 crystal the said minerals might enter into combination with the silica 

 and form niicrolites. It is not uncommon to see a fissure in a crystal 

 cemented with a brown iron oxide, which, if hydrous, would account 

 for a good deal of water. 



My collection of slices of vein-quartz is too small for any useful 

 purpose except to indicate possibilities ; but, so far as the slides go, 

 they are very instructive. Specimens collected between Dartmouth 

 and the Start contain one or more of such minerals as chlorite, epidote, 

 fibrous hornblende, and triclinic felspar. Then on Dartmoor and 

 its borders specimens of rock may be found showing every gradation 

 from the simple quartz-vein, through veins composed of quartz- 

 tourmaline and quartz-tourmaline-felspar (triclinic), to the quartz- 

 felspar-tourmaline rock of normal granitic structure. In all these 

 rocks the varying proportions of water and vacuity, and, above all, 

 the ubiquitous presence of dissolved salts, indicate a temperature well 

 under 342° C. One very eccentric slice from a vein in a greenstone 

 presents the appearance of a typical satiny quartz, but under the 

 microscope it is seen to contain innumerable water inclusions and 

 carbonic acid inclusions. It also contains a small crystal of plagio- 

 clase, and exhibits throughout the curved shadows of a felspar. 

 Observing that there seemed two varieties of quartz-veins near the 

 Bolt Head, I ascertained that one was very full of fluid inclusions 

 and the other unusually free from them. Possibly one represents the 

 era of metamorphosis, and the other may be either older or later. 



So far as I can ascertain, no one has worked the vein-quartz 

 problem, or at any rate has published the results. Dr. Sorby showed 

 in his first paper that the inclusions in quartz- veins in the neigh- 

 bourhood of granites were intimately connected with the inclusions 

 in the granitic quartzes themselves, but I am not aware that he has 

 anywhere described the differences between quartz-veins of different 

 ages and which have been subjected to different conditions. Nearly 

 all my quartz slides are post-Devonian, and possibly post-Carbon- 

 iferous, and they differ greatly from the quartzes in Culm grits and 



