part 1] ANNIVERSARY ADDRESS OF THE PRESIDENT. L\xi 



which is essential to the filling of mineral veins and to meta- 

 somatic changes like dolomitization, has no part in metamorphism 



proper, the fluid medium acting only as a solvent, not in general 

 as a carrier. It Avill be observed that I reject the much wider 

 definition of metamorphism adopted by Van Hise. The problem 

 would be greatly complicated, were we to include rocks which 

 are undergoing some arbitrary or unknown change in total 

 composition. 



Since the critical temperature is 374° for water and in general 

 much lower for the other volatile substances in question, the 

 solvent medium in any high grade of metamorphism must be 

 gaseous. Concerning the properties and chemical behaviour of 

 gases at high temperatures and under great pressures we possess 

 at present little information, though the systematic study of a 

 system involving silicate compounds with a volatile phase has 

 already been brought within the scope of laboratory methods. 

 Although I have used the words ' solvent ' and ' solution/ I do 

 not mean to imply that the action is of a merely physical kind. 

 Doubtless water in many cases and a fortiori the more active 

 substances take part in chemical reactions, which are of a cyclical 

 kind in this respect, that the solvent is released to repeat the 

 process upon a new portion of the material. How in this "way 

 a very small quantity of the solvent may serve to bring about 

 little by little the reconstitution of the whole mass, is well 

 illustrated in the lower grades of thermal metamorphism. Espe- 

 cially instructive are the ' spotted ' slates, which so often mark 

 the beginning of change in argillaceous sediments, since in these 

 we see the process arrested at an early stage. Such studies as 

 those of Hutchings make it clear that the spots (of this kind) 

 are merely patches of glass or partly devitrified glass. We see 

 that the process has begun at isolated points by the solution of 

 small portions of the rock-substance ; but, in this case, the re- 

 crystallization which should have followed has been checked by 

 fall of temperature. Vitrification of the rock in mass is found 

 only at an actual contact, where a more abundant supply of 

 solvent was furnished directly from its source, and is found there 

 only under conditions which ensured an intense heating and a 

 relatively rapid cooling. The intrusive rock in such a case is 

 always of thoroughly basic or ultrabasic nature, implying a high 

 temperature of intrusion, and it is a dyke or sheet of small 

 volume, which would soon cool down. 



