250 MINERALOGY 



increased by other components in the solution, especially 

 CO 2 , H 2 S, HC1, HF, or alkalies. 



All substances, even the most insoluble, are dissolved in slight 

 amounts ; insolubility is only a relative term. Gold itself is 

 soluble in water and especially so in the presence of ferric chloride, 

 and solutions of choloidal gold will retain the metal without sepa- 

 ration for a long time. Solubility is also increased with an increase 

 of pressure, provided that the total volume of solute and solvent 

 is decreased by solution. Hot flowing waters under pressure are 

 therefore very complex solutions, and are usually nearly saturated 

 with compounds which at the surface would be considered insoluble. 

 When such solutions, flowing through the veins and fissures of the 

 adjacent formation, gain access to regions of lower temperatures 

 and pressures, they are in a state of unstable equilibrium, and some 

 of their components are deposited. As the directional flow ' is 

 constant for long times, these streams of solutions serve to concen- 

 trate the soluble components in the fissures and veins, where they 

 are deposited, often in definite order and at times filling completely 

 the original veins and fissures along which the solutions flowed. 

 Many ore deposits filling veins, fissures, and pipes have been con- 

 centrated by this method ; and often where such heated solutions 

 reach the surface, as at the Steamboat Springs of Nevada, the depo- 

 sition of sulphides has been noted. Pyrite, chalcopyrite, galena, 

 arsenides, antimonides, and many minerals of like character have 

 been deposited on the walls of channels by the hot solutions flow- 

 ing through them. 



When minerals are crystallized under high pressures, the molecules 

 are compact, and the minerals formed are of a phase in which the 

 specific gravity is high ; as quartz, with a specific gravity of 2.65, will 

 form rather than tridymite, with a specific gravity of 2.3. Tridy- 

 mite occurs in surface lavas where it has crystallized at reduced or 

 atmospheric pressures. High pressures, other things being equal, 

 will induce the formation of pyrite, with a specific gravity of 5 

 rather than marcasite with a specific gravity of 4.9. As solids in 

 the crystalline phase occupy less space or have a higher specific 

 gravity than the amorphous phase, high pressures will tend to dis- 

 solve the amorphous phase and to redeposit the chemical com- 

 pound as crystalline. Large areas of amorphous limestones have 

 been thus crystallized under pressure. Ordinary obsidian when 

 crystallized will occupy less space and during crystallization will 

 shrink from 3 to 11 per cent, of its original volume. Garnets, feld- 



