1922] Whitman: Genesis of the Ores of the Cobalt District 305 



ions through the pore spaces of the rock, it would nevertheless be that 

 of mineral at 100 per cent concentration ; and, furthermore, it would 

 be directed from the point of its origin to the point of deposition by 

 the steepening of the diffusion gradient at the point where precipi- 

 tation is going on. 



Time seems to be the chief obstacle in the way of accepting diffusion 

 as a dominant agency in the genesis of metasomatic ore deposits. The 

 ratio of its effectiveness to that of water circulation is the point in 

 doubt. There are two forces which may be considered as tending to 

 actuate the circulation of water through the pore spaces of rock, capil- 

 larity and hydrostatic head, or difference in head. Capillarity oper- 

 ates only in the first wetting of the rock ; after that hydrostatic head 

 is the sole actuator. In the depths this can arise only from heat or 

 the compression of rocks containing water in sufficient quantity to 

 be squeezed out. In either case the head would have to be considerable 

 to drive water through capillary openings for a great distance ; and 

 the propagation of that force would be so slow that the passage of 

 large volumes of water past a certain point within a firm rock might 

 easily take as much time as the migration, by diffusion, of an equiva- 

 lent amount of mineral ions. This ratio of effectiveness is at present 

 indeterminable, and must await the results of experimentation. In 

 the meantime, however, we have some very definite chemical facts 

 in which to find assurance that diffusion is a factor to be reckoned 

 with. Fick, Soret, and others have made it clear that when a solute 

 is in contact with a solvent it tends to diffuse into the solvent until 

 equally distributed through it. The diffusion gradient consequent 

 upon Fick 's law supplies a directive agency and a means of accelera- 

 tion the moment precipitation begins within the field of diffusion. Its 

 operation is slow but relentless. The force actuating the diffusion of 

 salts has not been measured directly, but its supposed equivalent in 

 osmotic pressure has been equated 25 and measured, and found in cer- 

 tain instances to attain a magnitude of many atmospheres. Thus we 

 already know that where there is water and a solute, the latter 

 will tend to migrate through the former without cessation until equally 



25 Alexander Finlay in "Osmotic pressure,' ' p. 31, states the thermodynamic 

 equation for osmotic pressure as follows: 

 T"T 



P — V — Vo — [ — logc (1 — *) — % aP 2 ] . 



Here P = osmotic pressure, p = pressure of solution, p — pressure 'of solvent, 

 B — a constant depending upon the salt used, T — absolute temperature, I 7 = 

 molecular volume of solvent under standard pressure, x — molar fraction of the 

 solute, and a = coefficient of compressibility of the solvent. This is for concen- 

 trated solutions. 



