1922] 



Whitman: Genesis of the Ores of the Cobalt District 



291 



permit water to pass, then, due to the tendency of water to diffuse 

 into the solution, an endosmotic pressure will be built up, which may 

 reach a magnitude of several atmospheres. Now, if passages exist in 

 the chamber walls, which, while not large enough to permit the free 

 passage of the solute ions actuated by diffusion, are, nevertheless, large 

 enough to permit them to be dragged through by a current of water, 

 then, when the endosmotic pressure reaches a certain point, some of 

 the solution will leak out into the wall rock. In this manner, local 

 convection under a high osmotic head may be presumed to assist in 

 the dispersion of the solute. Probably in actuality the process would 

 not occur in stages, but osmotic pressure would be constantly cooper- 

 ating with diffusion, local obstructions to diffusion being frequently 

 overcome by the operation of osmosis. 



Fick's law that the rate of diffusion is proportional to the concen- 

 tration of the solute leads to the recognition of a diffusion gradient 

 relating concentration to the distance migrated in any given interval. 

 With such a gradient in mind it is easy to realize that after a consider- 

 able time a diffusing salt will have established a concentration gradient 

 such that its further migration will be at a very slow rate. However, 

 the moment the solution becomes impoverished in the solute at any 

 given point within the gradient, the gradient is at once steepened. 

 In this manner a point of precipitation within a diffusion gradient 

 not only causes the migration of solute particles to be directed toward 

 that point from all parts of the solution, but also accelerates diffusion 

 in that direction, and accelerates dissolution at the source of the solute 

 if that is a dissolving substance. Thus, if there are within a solution 

 a point of dissolution and a point of precipitation, there will be a 

 directed flow of matter from the former to the latter, both dissolution 

 and diffusion being accelerated by the presence of the precipitant. 



In the application of the principles of diffusion to earth phenomena, 

 it becomes necessary to consider the effects of heat and pressure upon 

 solubility, dissociation, diffusion, and precipitation. It is somewhat 

 hazardous to assume that the phenomena and the laws apparently 

 governing them within the narrow field of hrnnan observation continue 

 unaltered into the inaccessible regions where certain conditions are 

 known to be different to an unknown extent. The recognition and 

 projection of tendencies, however, is legitimate within certain limi- 

 tations; and in that sense it is probably safe to say that heat and 

 pressure tend to produce fortuity, while cold tends to produce differ- 

 entiation, discreteness, and order. Solubility and dissociation are in 



