484 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



are composed, or even through the pores existing in imperfectly 

 solidified metals, as the passage of water through solid glass would 

 differ from its passage amongst closely-packed fragments of glass. 

 Not only does this appear to me most improbable, but actually 

 opposed by facts. In the first place, the proportion between the 

 amount of fluid and the size of the cavities in the nepheline is so 

 uniform that I cannot believe it to be the result of accident, as we 

 should have to suppose if they were not all filled full at the same 

 temperature. If, to overcome this difficulty, it be supposed that the 

 fluid penetrated into the cavities when in a highly heated state, it 

 would require it to have been at the same temperature as that at 

 which I have supposed it entered in the same legitimate way that it 

 enters into the fluid-cavities in artificial crystals. But, even then, 

 the facts are against the supposition ; for, besides fluid-cavities, there 

 occur gas-cavities like figs. 86 and 87 ; and though there is no abso- 

 lute line of division between their form and that of the fluid-cavities, 

 their general characteristic shape is very different, because, as in 

 artificial crystals, in one case the crystal is moulded to the bubble of 

 gas, whereas in the other the irregular growth of the crystal deter- 

 mines the form of the cavity. Moreover, besides these gas-cavities, 

 there are the bubbles in the glass-cavities, which never contain a 

 fluid. If then we suppose that the fluid percolated through the 

 solid crystal into the fluid-cavities, we are led to conclude that it 

 selected these cavities like artificial fluid-cavities, but avoided those 

 resembling artificial gas-cavities, and the vacuities in the glass-cavities, 

 a conclusion which is so extremely unreasonable that we must reject 

 the proposition that leads to it. 



b. Granitic Rocks. 



In some of the trachyte of Ponza of solid character, as if it had 

 been formed under considerable pressure, there occur a few small 

 crystals of quartz, forming one of the genuine constituents of the 

 igneous rock, in every respect like those in many elvans and some 

 granites that contain but little quartz. They can scarcely be distin- 

 guished in the rock in its natural state, but are readily seen in a thin 

 section. When I examined this (April 1858), I found that the 

 quartz contains very excellent fluid-cavities, as shown by figs. 100, 

 101, and 102. There is no doubt that they contain a liquid, for the 

 bubbles move about in it. They are usually very flat, like fig. 101, 

 and, when inclined in particular positions, the transmitted light is 

 totally reflected from the bubble, which therefore appears like a 

 black opaque substance, as shown by fig. 102. By careful measure- 

 ments, I find that the relative size of the vacuities is very nearly 

 •30. Assuming then, that, like in the fluid-cavities in the minerals of 

 ejected blocks, and in the quartz of the veins in the self-same 

 trachyte, as well as in those in the quartz of elvans and granite, the 

 enclosed fluid is a strong aqueous solution of alkaline chlorides and 

 sulphates, I deduce, from equation (8), that the temperature at 

 which the crystals of quartz in the trachyte were formed was at 



