468 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



be difficult, or even impossible, to decide wbich process was essential, 

 or whether their combination was requisite for the production of any 

 particular kind of crystal. When, therefore, formed under great 

 pressure by the combined influence of liquid water and melted stone, 

 we may, I think, conclude that the crystals would contain glass- or 

 stone-cavities, and perhaps gas- and vapour-cavities, as well as fluid- 

 cavities, the relative size of the vacuities depending on the tempera- 

 ture and pressure, — whilst both the fluid- and vapour-cavities might 

 contain small crystals, deposited on cooling. 



§ 4. General Conclusions. 



The various facts described above will, I think, warrant the fol- 

 lowing general conclusions : — 



1 . Crystals possessing only cavities containing water more or less 

 saturated with various salts were formed by being deposited from 

 solution in water. 



2. The relative size of the vacuities in normal fluid-cavities de- 

 pends on the temperature and pressure at which the crystals were 

 formed, and may in some cases be employed to determine the actual 

 or relative temperature and pressure. 



3. Crystals containing only glass- or stone-cavities were formed by 

 being deposited from a substance in the state of igneous fusion. 



4. Crystals containing only gas- or vapour-cavities were formed by 

 sublimation or by the solidification of a fused homogeneous sub- 

 stance, unless they are fluid-cavities that have lost all their fluid. 



5. Other circumstances being the same, crystals containing few 

 cavities were formed more slowly than those containing more. 



6. Crystals possessing fluid-cavities containing a variable amount 

 of crystals, and gradually passing into gas-cavities, were formed 

 under the alternate presence of the liquid and a gas. 



7. Crystals in which are found both cavities containing water and 

 cavities containing glass or stone were formed, under great pressure, 

 by the combined action of igneous fusion and water. 



8. Crystals having the characters of 6 and 7 combined were 

 formed, under great pressure, by the united action of igneous fusion 

 and water alternating with vapour or a gas, so as to include all the 

 conditions of igneous fusion, aqueous solution, and gaseous sub- 

 limation. 



Such then are the general principles I purpose to apply in investi- 

 gating the origin of minerals and rocks. It will be perceived at once 

 that, in one way or other, they may be brought to bear on almost 

 every branch of physical and chemical geology. In this communi- 

 cation I shall illustrate the subject by applying them to some of the 

 leading branches of inquiry, without attempting to treat each in a 

 complete manner. 



II. Structure of Natural Crystals. 



§ 1 . Methods employed in examining minerals and rocks. 



In examining the microscopical structure of rocks and minerals, I 

 have in many cases prepared sections sufficiently thin to admit of 



