ANNIVERSARY ADDRESS OE THE PRESIDENT. 



79 



investigations like those of Hague and Iddings in the Comstock 

 region, and of Lotti in Elba. Further and more recent researches 

 have enabled me to show that certain types of structure have been 

 determined in rocks, according to the more or less perfect absence 

 of all movement within them during their consolidation. 



Yery remarkable, indeed, are the internal changes which take place 

 in rock-masses when they are submitted to those powerful stresses 

 which result from the movements that occur during mountain- 

 making. 



It was long ago asserted by Scrope and Darwin that the solid 

 rock-masses of the globe, under such conditions as these, must 

 have actually flowed, like the viscous lavas of the rhyolitic series. 

 They were even able to show that the separation and disposition of 

 the crystalline elements in such lavas present the closest analogy 

 with what is seen in the crystalline schists and gneisses of greatly 

 disturbed areas. 



Since these early and notable researches, which were principally 

 based on the study of rocks in the field, aided only by the pocket- 

 lens, three classes of investigations have served to deepen our insight 

 into the methods by which the schistose and gneissose rocks must 

 have been produced. 



In the first place, the experiments of MIT. Tresca and Daubree 

 have shown that solid matter under enormous pressure behaves like 

 a viscous substance, its whole internal structure exhibiting evidence 

 of the flowing movements to which it has been subjected. 



In the second place, the studies of M. Spring have established the 

 fact that both paramorphic change and direct chemical reaction 

 may result from simple pressure. Thus the unstable monoclinic 

 form of sulphur, by a pressure of 5000 atmospheres, is at ordinary 

 temperatures instantly converted into the stable rhombic form, a 

 transformation accompanied by change of density and of many other 

 physical properties. Still more striking is the well-known case of 

 the unstable, yellow, rhombic, mercuric-iodide, which, by simple 

 rubbing with a hard substance, passes into its stable, red, tetragonal 

 allomorph. It is instructive to notice that the same change in both 

 instances appears to take place " spontaneously " after a sufficient 

 interval of time ; or, in other words, small variations in tempera- 

 ture, pressure, and other surrounding conditions are capable, if 

 sufficient time be allowed, of bringing about the same result as more 

 intense pressure applied suddenly. That the similar paramorphic 

 change of pyroxene into hornblende, which is so frequently exem- 



