178 OEIGIN AND DEVELOPMENT OF OEE-DEPOSITS. 



2. — Action of molecular ajfinity and of the crystallizing forces. 

 The tendency, everywhere observable in mineral substances, of 

 like particles to approach each other whenever freedom of motion 

 is permitted, seems to depend upon an affinity of the molecules 

 which is quite distinct from what is known as chemical affinity — 

 this latter being strongest between unlike molecules. 



Molecular affinity seems to be the originating agent in the 

 production of concretionary structure in colloids, and of sporadic 

 crystallization, the devitrification of glassy rocks, and crystalliza- 

 tion generally in crystalloids. The subsequent development of 

 crystalline structure in colloid aggregations is a related but 

 distinct phenomenon which is also frequent. 



To enable this molecular affinity to operate, freedom of 

 motion of the particles is, of course, necessary. This exists 

 originally in both eruptive and sedimentary rocks to a very 

 considerable extent. In eruptive rocks fusion, or the pseudo-fusion 

 due to the presence of moisture at a very high temperature, 

 facilitates the formation of such concretionary aggregates, 

 mostly more or less crystalline, as occur in the granites of the West 

 of England,^' and also of the sporadic crystallization generally, 

 which results in what is called " porphyritic " structure. This 

 may be on a large or small scale — "macro"- or " micro "- 

 porphyritic. The separate crystals or crystal-groups are in some 

 cases developed in a glass in a cryptocrystalline base, as in the 

 case of most of the " elvans ;" or else in a well crystallized granitic 

 base, as may be seen in much of the granite in all parts of the 

 two western counties. 



In stratified rocks such as clays, mudstones, sandstones and 

 limestones, concretions of ferruginous, phosphatic, or siliceous 

 (cherty) matter of a colloid character ; cubes of pyrites ; crystals 

 and crystal-groups of gypsum and other crystalline minerals 

 are often thus formed before complete consolidation. Such are 

 good examples of the action of " molecular affinity " in bringing 

 together like particles of matter already existing in a diffused 

 condition throughout the mass. I proceed to refer to a few 

 examples in detail. 



*Phillips, Quart. Juur. Geol. Soc, xxxvi, p. 1 and xxxvii, p. 216. 



