CXXV111 PROCEEDINGS OP THE GEOLOGICAL SOCIETY. 



the examination of the " Microscopical structure of Crystals, with a 

 view to the determination of the Aqueous or Igneous origin of Minerals 

 and Rocks," which is one of the most abstruse and difficult questions 

 in physical geology. As a general rule, it must of course be admitted 

 that the molecules of any substance must be endowed with a power 

 of movement before they can arrange themselves into any definite 

 form ; and, as regards mineral substances, it has therefore been always 

 considered necessary that they should be in a state of solution before 

 the elementary constituents, of which they are composed, could re- 

 arrange themselves into a definite crystalline form. Taking, in the 

 first place, notice of artificial crystals, since the whole process can in 

 them be brought under actual observation, Mr. Sorby points out that, 

 though the cohesive force of the particles is reduced to a minimum, 

 it is not absolutely destroyed, and hence that, in the act of cry- 

 stallizing, portions of the solvent surrounding them at the time of 

 formation would be often caught up and enclosed within their solid 

 substance. When, therefore, crystals are produced by sublimation, 

 either air or vapour is imprisoned, which, on being condensed by cold, 

 leaves apparently empty cavities or air-cavities ; when the crystalli- 

 zation is from an aqueous solution, fluid-cavities are formed; when 

 from an igneous solution, the crystals which separate themselves from 

 the fused-stone solvent may be expected to catch up and entangle in 

 their substance some portions of the mineral bath, which on cooling 

 resume their original character, and produce what may be called glass- 

 or stone-cavities. The differences between these several forms of ca- 

 vities can be readily distinguished with suitable magnifying powers, 

 and thereby afford the means of determining under what conditions 

 the crystals had been formed : as, for example, crystals containing only 

 fluid- cavities, from aqueous solution ; crystals containing only stone- 

 or glass-cavities, from igneous fusion ; crystals containing both water- 

 cavities and stone- or glass-cavities, from the combined influence, under 

 great pressure, of highly-heated water and melted rock : and, further 

 than this, that in the case of fluid- cavities, the amount of water pre- 

 sent affords a datum for determining, from the amount of condensation 

 it appears to have undergone since the original formation of the cavity, 

 the temperature it possessed when entirely filling the cavity. In like 

 manner, Mr. Sorby considers that empty cavities indicate that the cry- 

 stals containing them have been formed by sublimation, unless there 

 is reason to believe that the enveloping matter was sufficiently porous 

 to allow the imprisoned fluid to escape, or that the cavities were 

 merely bubbles due to fusion. The number of cavities may be ex- 

 pected to increase with the rapidity of crystallization, whilst a total 

 absence of cavities indicates either very slow crystallization, or the 

 cooling of a fused homogeneous substance. These general principles 

 are then applied by Mr. Sorby to the study of natural crystalline 

 minerals and rocks ; and he deduces from them many highly interest- 

 ing results: for example, that the fluid-cavities in rock-salt, in the 

 calcareous spar of modern tufas, in vein-stones, in ordinary limestone, 

 and in gypsum, indicate that these minerals were formed by deposition 

 from solution in water, at a temperature not materially different from 



