MOVEMENTS AND DEFORMATIONS OF THE EARTH'S BODY. 555 



ma)^ be regarded as a specialized variety of cohesion which usually 

 cooperates with gravity to produce increased density. In cases of 

 expansion it seems clear that the organizing force does not act according 

 to the law of gravity. The intensity of the force exhibited in the for- 

 mation of ice illustrates the superiority of the molecular force over the 

 gravitative force in small masses; but in a planet of ice of very mod- 

 erate dimensions, the internal pressure of gravity would overcome the 

 crystalline force, which illustrates the superiority of gravity in large 

 masses. 



While the crystalline force may thus in exceptional cases operate 

 against gravity, it is known that in most cases it not only operates with 

 it, but is controlled by it, in this sense — that where a substance has two 

 forms of crystallization, it will take the denser one when the pressure 

 is great. The inference is that if the less dense form of crystallization 

 takes place under slight pressure, and subsequently the pressure is 

 greatly increased, the form of crystallization will change from the less 

 to the more dense/ It is probable that in general those forms of molec- 

 ular arrangement will be assumed in the deep zones which give the 

 greatest density, and this probably includes concretionary, colloidal, 

 and other forms of aggregation, as well as crystallization. 



Diffusion. — The same law probably holds relative to diffusion, though 

 in a molecular sense diffusion is the opposite of crystallization, for in 

 crystallization, like comes to like, while in diffusion the molecules distrib- 

 ute themselves among those of unlike nature. Diffusive action, quite 

 familiar in gases and liquids, takes place to some extent in solids. The 

 molecules of plates of gold and lead brought into intimate contact under 

 pressure mutually diffuse among one another. So gases seem to be 

 very generally diffused or "occluded" in rocks, though the nature of 

 this relation is imperfectly determined. It is known that pressure upon 

 gases promotes their diffusion through hquids and solids. It is inferred 

 that pressure upon a solid tends to the diffusion of the entrapped gases 

 within it, but it is not to be inferred from this that pressure upon rock 

 promotes the absorption of gases into it, but rather the opposite. It is 

 probable that great pressure with high heat promotes the diffusion of 

 entrapped gases or other diffusible substances through the rock -mass, 

 and at the same time tends to their extrusion along lines of least resist- 

 ance; but this is an inference rather than a demonstration. 



1 Van Hise. Bull. Geol. Soc. of Am., 1897, Vol. IX, p. 291. 



