part 1] ANNIVERSARY ADDRESS OF THE PRESIDENT. lxix 



from which they come. In these, therefore, metamorphism may 

 be supposed roughly to start from equilibrium, and successive 

 reactions will come into play as the temperature rises, subject 

 only to a greater or less lag-effect. Accordingly, it is in rocks of 

 this type, and in them only, that we can trace from the beginning 

 successive zones of metamorphism, characterized by the first 

 appearance of distinctive new minerals. In sediments of mixed 

 origin no such nicely-graduated series of stages is observable. 

 In particular, we see here the reason of the peculiar behaviour 

 of partly calcareous sediments, which afford a typical example 

 of ' false equilibrium ' as between the two kinds of constituents, 

 silic'ate and carbonate. Here, so soon as with rising temperature 

 the rate of possible reactions becoiues sensible, the reactions 

 proceed with a rapidity limited only — since they are endo- 

 thermic — by the supply of heat. It is well known that in the 

 field the transition from a seemino-lv unaltered sediment of 

 this type to a rock composed wholly of new minerals is often 

 strikingly abrupt, and, again, beds of this nature intercalated 

 among simple argillaceous sediments, still in the first stages of 

 alteration, afford the best examples of the so-called ' selective ' 

 metamorphism. The relatively low temperature at which complete 

 reconstruction is effected sets very narrow limits to the inter- 

 change of material within the rock, and the result is seen in 

 the exceedingly fine texture Of many lime-silicate rocks originating 

 in this way. The complex mineralogical constitution of man}' 

 of these rocks is significant in the same sense. It is easy to 

 believe that total reconstruction so hastily effected does not 

 always result in a theoretically stable association of products, 

 and the number of distinct minerals found in some of these lime- 

 silicate rocks is clearly in excess of that allowed by Goldschmidt's 

 rule. 



The recrystallization of rock-masses and the various che- 

 mical reactions comprised in their metamorphism all presuppose 

 solution of the material; and the part played by solvents 

 becomes therefore of fundamental importance in the chemistry 

 of metamorphism. The nature of t. 1 e most common solvents is 

 scarcely in doubt. It is sufficiently indicated by examination of 

 the metamorphosed rocks themselves, where some part of these 

 substances has often entered finally into the composition of 

 certain of the new minerals. Water is doubtless the most 



