8o PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



plified in the earth's crust, is sometimes the result of intense 

 pressure, and at other times follows from the repeated slight altera- 

 tion of conditions during long periods of time, we have, I believe, 

 abundant evidence. 



The experiments of M. Spring which prove that direct chemical 

 reactions can result from the action of pressure, are, however, of 

 even greater interest to the geologist. By submitting mixed powders 

 to intense compression, he succeeded in producing metallic alloys 

 and various binary compounds, and also in bringing about double 

 decomposition between many salts. That similar reactions between 

 the complicated silicates which form the minerals of rocks have 

 resulted from the enormous pressures to which they have been sub- 

 jected, we have the most ample proof. Thus in rocks where such 

 pressure has just begun to act, such as the " flaser-gabbros," 

 wherever the unstable olivine is in contact with the almost equally 

 unstable anorthite, chemical reactions have been set up by the 

 pressure, and these have resulted in the formation of zones of 

 enstatite and anthophyllite, hornblende and biotite, which have 

 been so well described by Tornebohm, Bonney, Adams, and "Williams 

 Provided with the clue supplied by these results, we find little 

 difficulty in going one step further. "When the pressure has been 

 still more intense, as in mountain-making movements, reactions 

 are set up among all the minerals of the rock-mass, the elements 

 of which it is composed, set free from their old engagements, enter 

 into new alliances, and the result is the formation of a completely 

 new set of crystallized minerals. 



The third class of researches, destined, as I believe, to remove our 

 difficulties in explaining the origin of the schistose and gneissose 

 rocks, are those already alluded to as having been undertaken with 

 the microscope. As yet the details of such changes have only been 

 explained in the case of some of the simpler examples ; but I am 

 convinced that the persevering application of the same methods in 

 the field and the laboratory will result in the removal of difficulties 

 that now seem to be absolutely insuperable. 



Some observers in this country have been led to infer that the 

 recrystallization of rock-masses under pressure has in all cases been 

 preceded by their pulverization. Of this, I confess that I can find 

 no evidence. That near great faults of all kinds this reduction of 

 rocks to powder does take place, we find abundant proof ; but the 

 evidence seems to me to also point to the conclusion that such 

 rock-crushing, as distinct from rock-flowing, is in every case local 

 and exceptional. 



