SORBY — STRUCTURE OF CRYSTALS. 485 



least 356° C, which closely corresponds with the mean deduced 

 from the fluid-cavities in the blocks ejected from Vesuvius. At this 

 temperature the elactic force of the vapour of water is equal to about 

 2400 feet of rock, and therefore the quartz must have crystallized 

 under that pressure, at least. Considering the nature of the rock, 

 the pressure cannot, I think, have been very much more than that, 

 though it must have been somewhat more, and then of course the 

 calculated temperature would be higher ; but it would require a 

 pressure equal to upwards of 19,000 feet of rock to alter it to 400°O. 

 If it was equal to about 4000 feet, the calculated temperature would 

 be 360° C. (680° F.) . This is a very dull red heat visible in the dark, 

 and the elactic force of the vapour of water would be equal to 2500 

 feet of rock. It does not necessarily follow that the rock was finally 

 consolidated under such a pressure, or at such a depth, since the 

 strength of quartz is such, that, if the crystals had been formed at a 

 considerable depth, they might be carried to a much less without the 

 elastic force of the fluid bursting the cavities. To completely fuse 

 such a rock, a white heat is necessary ; but I find that, when in a 

 glassy state, thin fragments become soft enough to bend at a very 

 moderate red heat, so that the temperature at which it became quite 

 solid probably could not differ very materially from the dull red heat 

 deduced from the fluid-cavities, the two independent facts strongly 

 confirming each other. 



Along with these fluid-cavities occur most excellent stone-cavities, 

 as shown by fig. 103, in every respect analogous to those in the 

 crystals in slags, and especially like some in leucite; and it may 

 easily be seen that they are small portions of the surrounding fel- 

 spathic material of the trachyte, that have been enclosed in the 

 growing crystals of quartz. That they were caught up when their 

 substance was in a fused, or at all events in a soft state, is proved by 

 the fact, that their form is related to, and they are moulded upon, 

 the crystalline planes of the quartz ; whereas, if they had been solid 

 fragments, the quartz would have been moulded to their own form. 

 It therefore appears to me to be completely proved, that these crystals 

 of quartz were generated under similar physical conditions to those 

 concerned in the development of the minerals of the ejected blocks, 

 by the combined influence of a dull red heat, liquid water, and par- 

 tially melted rock. 



The structure of the quartz of many elvans and some granites is 

 in every respect analogous to that in the trachyte just described. 

 The only sensible difference is that the fluid-cavities are seldom so 

 flat, and gas- or vapour-cavities more numerous. The proof of the 

 igneous origin of elvans is complete, for the stone-cavities are very 

 well developed. Examples of these are shown by figs. 104, 105, 

 106, and 107, from the elvans near Penrhyn and Gwennap. As will 

 be seen, fig. 104 is extremely like those in the trachyte, differing 

 only in being of rather coarser grain, and in containing a long prism 

 of schorl. Very often long hair-like crystals of that mineral occur 

 in the quartz itself, sometimes attached to stone-cavities, as shown by 

 fig. 106 ; like the crystals with attached stone-cavities in leucite, 



