486 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



fig. 67 > Fig. 105 represents a cavity of more irregular shape, and 

 fig. 107 is from an elvan of much coarser grain, and has a gas- or 

 vapour-cavity attached to it. These gas- or vapour-cavities often 

 occur in distinct bands, like those in augite and other volcanic 

 minerals ; but sometimes they are mixed up with fluid-cavities, which 

 in that case have bubbles of variable relative size, caused by the 

 irregular combination of fluid and vapour in the same cavity ; whilst 

 in other parts fluid-cavities occur alone, with vacuities of very uniform 

 relative size. As in the case of artificial crystals, the form of the 

 fluid- cavities is often related to the crystalline planes of the quartz, 

 as shown by fig. 108. Fig. 109 is a fluid-cavity containing prismatic 

 crystals, which in some cases must certainly have been caught up 

 during the growth of the quartz, and not deposited from the solution 

 on cooling, for they often pass through the cavities, as shown by fig. 

 110, and appear to be schorl. 



The passage from elvans to granite is quite gradual, and this is 

 also the case with the peculiarities in the microscopical structure of 

 their constituent minerals. The quartz of granite often abounds 

 with most excellent fluid-cavities, and as an illustration of this fact, 

 I have in fig. Ill represented a portion of the quartz of the granite 

 of St. Austel, which occurs as distinct crystals, precisely like that in 

 the trachyte described above, and not as a residue of crystallization. 

 In many granites the fluid-cavities are so numerous in the quartz, 

 that on an average they are not above y^y^-th of an inch apart. This 

 agrees with the proportion of a thousand millions in a cubic inch ; 

 and in some cases there must be more than ten times as many. 

 They also really constitute a most important part of the whole bulk 

 of the quartz, for sometimes they make up at least 5 per cent, of the 

 volume ; and I have found that the loss of water on heating the 

 quartz of the granite of Cornwall to redness is on an avt r Age about 

 # 4 per cent, of its weight, which is equivalent to about 1 p. c. of its 

 bulk. These fluid-cavities are not confined to veins of granite, or to 

 that part near the junction with the stratified rocks, but are quite as 

 numerous in the most solid rock, far away from the junctions ; as 

 though the fluid was not an accidental ingredient, due to the perco- 

 lation of water to a fused mass naturally containing none, but as if it 

 was a genuine constituent of the rock when melted. Their number 

 varies very much in different granites, but hitherto I have found 

 them in all specimens I have examined ; and, though there are ex- 

 ceptions to the rule, yet on the whole they are more numerous in 

 granites than in elvans, and in coarse-grained, than in fine-grained 

 granites*. 



The felspar of the Cornish granites is usually so opaque, on account 

 of partial decomposition, that it is difficult to see the fluid-cavities. 



* This is especially the case near Aberdeen ; for in the quartz of the coarse- 

 grained veins, having en stals of mica, felspar, and schorl several inches long, the 

 fluid-cavities are so numerous, large, and distinct, that even with only a mode- 

 rately high magnifying power they may be seen to greater advantage than in any 

 other granite I have hitherto examined ; whereas in the rather fine-grained stone 

 used in building, they are few, small, and ohscure. — Oct. 1858. 



