Mr. Sorby on the Microscopical Structure of Crystals. J 53 



were formed, under great pressure, by the combined influence of 

 highly heated water and naelted rock. 



4. That the relative amount of water present in the cavities may, 

 in some cases, be empbyed to deduce the temperature at which the 

 crystals were formed, since the accompanying vacuity is due to the 

 contraction of the fluid on cooling. 



5. Crystals containing only empty cavities were formed by sub- 

 limation, unless the cavities are fluid-cavities that have lost their 

 fluid, or are bubbles of gas given off from a substance which was 

 fused. 



6. Crystals containing few cavities were formed slowly, in com- 

 parison with those of the same material that contain many. 



7. Crystals that contain no cavities were formed very slowly, or by 

 the cooling from fusion of a pure, homogeneous substance. 



_ Applying these general principles to the study of natural crystal- 

 line minerals and rocks, it was shown that the fluid-cavities in rock- 

 salt, — in the calcareous spar of modern tufaceous deposits, of veins, 

 and of ordinary limestone,— and in the gypsum of gypseous marls, 

 indicate that these minerals were formed by deposition from solution 

 in water at a temperature not materially different from the ordinary. 

 The same conclusions apply to a number of other minerals in veins 

 in various rocks, and to many zeolites. The constituent minerals of 

 mica-schist and the associated rocks contain many fluid- cavities, in- 

 dicating that they were metamorphosed by the action of heated 

 water, and not by mere dry heat and partial fusion. 



The structure of the minerals in erupted lava proves that they 

 were deposited from a mass in the state of igneous fusion, like the 

 crystals in the slags of furnaces; but, in some of those found in 

 blocks ejected from volcanos (for example, in nepheline and meionite), 

 there are, besides stone- and glass-cavities, many containing water, 

 the relative amount of which indicates that they were formed, under 

 great pressure, at a dull red heat, when both liquid water and'melted 

 rock vvere present. The fluid-cavities in these aqueo-igneous minerals 

 very generally contain minute crystals, as if they had been deposited 

 on cooling from solution in the highly heated water. The minerals 

 in trappeau rocks have also such a structure as proves them to be of 

 genuine igneous origin, but they have been much altered by the sub- 

 sequent action of water, and many minerals formed in the minute 

 cavities by deposition from solution in water. 



The quartz of quartz-veins has a structure proving that it has been 

 rapidly deposited from solution in water ; and in some instances the 

 relative amount of water in the fluid-cavities indicates that the heat 

 was considerable. In one good case the temperature thus deduced 

 was 1G5° C. (.329° F.); and apparently, when the heat was still 

 greater, mica and tinstone were deposited, and in some cases pro- 

 bably even felspar. 'I'here is, then, as has been argued by M. Elie 

 de Beaumont, a gradual passage from quartz-veins to tliose of granite, 

 and to granite itself; and there is no such distinct line of division 

 between them as might be expected if one was a deposit from water, 

 and tiie other a rock that had been in such a state of jjure igneous 



