VOLUME DECREASED BY RECRYSTALLIZATION. 103 



found in the volcanic rocks. The plutonic rocks crystallize under condi- 

 tions of great pressure, while the volcanic rocks crystallize under conditions 

 of moderate or slight pressure. It would be interesting to know the 

 relations of quartz and tridymite in the matter of depth in the lavas, and 

 therefore in reference to pressure at the time of crystallization. 



SMALLER VOLUME AS THE KESOLT OF SOLUTION AND KEDEPOSITION WITH CHANGE IN CHEMICAL COMPOSITION. 



Pressure inducing chemical reactions involving changes in chemical 

 composition may produce crystallization and condensation of amorphous 

 compounds and recrystallization and condensation of crystallized com- 

 pounds. 



Crystallization and condensation of amorphous compounds. 111 O'elieral tile amOTOhoilS 



compounds occupy more volume than their complex crystalline equivalents. 

 "Therefore, since the crystallized state is generally that which takes the 

 smallest volume, pressure aids crystallization." a According to Delesse, in 

 passing from the crystalline to the glassy state, granite decreases in density 

 9 to 11 per cent, syenite 8 to 9 per cent, diorite 6 to 8 per cent, dolerite 5 

 to 7 per cent, and trachyte 3 to 5 per cent.'' Thus glass occupies from 3 to 

 11 per cent more volume than the equivalent crystallized rocks. It there- 

 fore follows that pressure is one of the potent forces which result in the 

 devitrification of glass. In general it may be said that rocks near the 

 surface, whether original magmas, sediments, or schists and gneisses partly 

 altered in the belt of weathering, very frequently contain amorphous prod- 

 ucts; whereas rocks which have been altered while deeply buried rarelv 

 contain any considerable quantity of amorphous material. It is believed 

 that the explanation of the difference is largely due to difference in pres- 

 sure. At depth where pressure is forceful the amorphous products which 

 occupy more space than their crystallized equivalents either have not 

 formed or if formed at the surface and deeply buried have become crystal- 

 lized, the pressure being one of the important forces in the process. 



Recrystallization and condensation of crystallized compounds. .P reSSUl'e UiaV induce cllem- 



ical action upon crystallized compounds, producing reciystallized products 

 of a different kind and with more compact molecules, and therefore of 

 greater specific gravity. In some cases the recrystallization has occurred 



«Tolman, C. F., jr., Professor Spring on the physics and chemistry of solids: Jour. Geol., vol. 6, 

 1898, p. 320. 



6 See Dana, J. D., Manual of geology, American Book Co., 4th ed., 1895, p. 265. 



