184 ORIGIN AND DEVELOPMENT OF OBE-DEPOSITS. 



In many cases, where original components of the rocks are 

 in question, the freedom of motion existing in the rock-masses 

 before complete consolidation would be sufficient to allow of the 

 formation of crystal aggregates of considerable size. In other 

 cases, so evenly balanced are the aggregating and retaining 

 forces that the presence or absence of actual cavities is sufficient 

 to determine whether there shall be such aggregations or not. 

 But we know that mineral growth is possible, and indeed is 

 often effected in rock-masses after complete solidification. The 

 necessary freedom of motion, and the spaces for the new aggre- 

 gations are provided slowly but simultaneously, — the solvent 

 powers of the circulating solutions being aided by heat and 

 pressure, and the aggregating tendencies by electricity, surface- 

 tension, and, sometimes, by direct chemical reactions. So 

 continuously, though slowly, do these changes go on, that, in 

 some instances, well-formed crystals of large size have been 

 formed around suitable nuclei in already solidified rock-masses, 

 as shewn by Judd, and others.*" Even the pressures resulting 

 from rock-movements may be thus effective, by controlling the 

 chemical affinities and crystallizing forces. 



* Q.J .G.S., 1889, p. 175. Mr. Judd found that detrital fragments of quartz, 

 felspar, hornblende, and mica had become enlarged under these conditions, and 

 that crystals of many kinds had also been enlarged, those of felspar even after 

 partial kaolinization. 



[7b he continued.'] 



