ANNIVEESAKY ADDRESS OF THE PEESIDENT. 9^ 



ore. That similar processes can go on in nature is suggested by 

 the numerous instances which we witness in studying the conversion 

 of olivine-rock into serpentine, of pyroxene into varieties of horn- 

 blende, the formation of tourmaliue in granite, and the like. I 

 believe also that many of the oolitic grains in limestones are struc- 

 turally true " spherulites " developed after the rock was more or less 

 consolidated ; and that such is the nature of the radiate balls in the 

 magnesian limestones of Durham can hardly be doubted. Many other 

 instances of " concretion " might readily be mentioned, notably those 

 singular forms in the flinty slates of Eskdale : but the above may suffice. 

 I formerly pointed out that the spherulitic structure of certain felsites 

 in Arran could only be explained on the supposition that they had 

 been produced by a metamorphic action due to a subsequent intrusion 

 of an igneous rock. The structures of the de vitrified glasses also 

 show us very clearly how great an influence the slightest disturbance 

 of equilibrium has on the initiation and direction of crystal-growth. 

 The discontinuity, and consequent diff'erence, due to the mere 

 existence of a surface — what we may term the surface tension — has 

 sufficed to originate crystallization in each one of these cases of 

 artificial secondary devitrification. 



Thus my examination of a large number of igneous rocks in the 

 light obtained from the experimental evidence described above leads 

 me to the following conclusions : — 



(1) That spherulitic and other microlithic structures can be 

 produced in a glassy rock during cooling. 



(2) That they may sometimes originate from a nidus (as it were) 

 of slightly different mineral composition, which thus starts crystal- 

 lization. 



(3) That they very often originate by the mechanical aid of some 

 included crystal or particle. 



(4) That perhaps still more often they are the result of some kind 

 of strain analogous to that of the artificial cases described above. 

 This might occur especially in banded rocks, as the difference in 

 composition in the layers might cause them to contract unequally. 



(5) That these microlithic structures, unless too crowded, are 

 sharply separated from the surrounding glass. 



(6) That they can also be produced by subsequent heating short 

 of fusion, and that, except perhaps that the results are more obviously 

 connected with local disturbance of equilibrium, there are no means 

 of distinguishing between " dry heat " devitrification and " slow cool- 

 ing " devitrification. 



But the experiments of Prof. Daubree have produced results not 

 wholly identical with those of the drj- -heat action ; and to this ex- 

 periment the process which has taken place in nature must have 

 been more nearly analogous ; that is to say when " devitrification," in 

 the strict sense of the word, has been produced in a rock once glassy, 

 the agents of change have been pressure, water, heat, the elevation of 

 temperature being probably in most cases very moderate. How far, 

 then, is it possible to distinguish the results of this from those of 

 "cooling devitrification," the only other kind likely to occur, except 

 very locally, in nature ? 



VOL. XLI. h 



