SORBY — STRUCTURE OE CRYSTALS. 475 



and that associated with chalcedony in veins or in cavities contains 

 very few fluid-cavities with relatively small vacuities, indicating a slow 

 deposition from water at a much lower temperature. 



§ 4. Metamorphic Rocks. 



In some portions of the granite containing large crystals of felspar 

 at Trevalgan, near St. Ives, Cornwall, the felspar has been more or 

 less completely removed, and its place filled with quartz, mica, or 

 schorl, either alone or variously mixed. These most interesting and 

 important pseudomorphs appear to have been almost entirely over- 

 looked ; though I have found them in so many other localities in Corn- 

 wall that they cannot be very rare. The removal of the felspar from 

 the centre of the surrounding, fine-grained granite, and the introduc- 

 tion of the quartz, mica, and schorl, cannot I think be explained 

 except by the action of water. In this quartz are many very interest- 

 ing fluid-cavities, and in some parts nearly all contain small cubic 

 crystals, as shown by fig. 53. In other cases, besides such cubes, 

 there occur prismatic crystals, like in fig. 55, or more rarely rhombic, 

 as fig. 54. Occasionally the angles of the cubic crystals are corroded 

 and rounded, as shown by fig. 55 ; and some cavities, as fig. 56, are 

 so full of crystals that their form cannot be determined. The quartz 

 also contains gas- or vapour-cavities, and every connecting link be- 

 tween them and the other cavities. In all respects therefore the 

 structure of this quartz is analogous to those crystals that are formed 

 artificially above the surface of a hot liquid, and exposed alternately 

 to water and air. When reduced to powder, water dissolves out much 

 chloride of sodium, and a good deal of sulphate of lime, and hence 

 the cubic crystals in the fluid-cavities are no doubt chloride of sodium, 

 and perhaps some of the prisms may be selenite. Even if the effects 

 of pressure are supposed to have been not material, the relative size 

 of the vacuities indicates a heat of 220° C. ; but, since the relative 

 size of those in the fluid-cavities in the granite is nearly the same, in 

 accordance with the principles described below, the pressure was pro- 

 bably very great, and the temperature nearly or quite equal to a dull 

 red heat, visible in the dark. 



It therefore appears, that to the action of water at a very high 

 temperature, holding various salts in solution, must be ascribed the 

 removal of the felspar, and the production of the mica, quartz, and 

 schorl. In a paper read at the British Association (Report, 1857, 

 p. 92), I showed that the material of the quartz and mica might be 

 derived from felspar, decomposed by the removal of part of the alka- 

 line bases ; and we thus have a key to those cases of metamorphosis 

 where deposits of decomposed felspar-clays have been converted into 

 crystallized mica and quartz, so as to constitute mica-schist. In the 

 bands of quartz in mica-schist and gneiss, which are as it were irre- 

 gular concretions passing along the foliation, and in the carbonate of 

 lime and iron sometimes associated with the quartz, occur vast num- 

 bers of fluid -cavities containing water. The quartz mixed up with 

 the mica, forming the chief constituent of the schist, also abounds 

 with fluid- cavities ; and I have even found them in some of the 



VOL. XIV. — PART I. 2 I 



