METAMORPHISM OF ROCK-MA SSES. 5 7 5 



Metasomatic changes are known to take place during thermal 

 metamorphism as regards the volatile constituents of the rocks 

 affected. A (usually partial) loss of water and the elimination 

 (under proper conditions) of carbonic acid from carbonates are 

 instances of this ; a more special case is the accession of boric 

 and hydrofluoric acids near the contact of metamorphosed rocks 

 with certain acid intrusives. Several observers have recorded a 

 transference of other materials (silica and soda) from an invad- 

 ing igneous magma to the neighboring rocks, but such a phe- 

 nomenon seems to be of uncommon occurrence, and to be con- 

 fined to the immediate vicinity of the contact. Apart from the 

 exceptions noted, there is every reason to believe that thermal 

 metamorphism involves no alteration in the bulk-analysis of the 

 rocks affected. Whatever part water may play in the various 

 chemical changes that are set up, it does not (as in atmospheric 

 metamorphism) act as a medium to transfer material to or from 

 the rocks in question. 



I believe that we can go further, and assert that within the 

 mass of a rock undergoing thermal metamorphism any transfer- 

 ence of material (other than volatile substances) is confined to 

 extremely narrow limits, and consequently that, for a given tem- 

 perature of metamorphism, the mineral formed at any point 

 depends only on the chemical composition of the rock-mass 

 within a certain very small distance around that point. Illustra- 

 tions of this principle, as stated in the latter form, are familiar to 

 all who have studied cases of "contact metamorphism" : x they 

 are very striking when some of the constituent substances of the 

 original rock were, by weathering or otherwise, locally aggregated 

 prior to metamorphism. By studying such cases we can not only 

 verify the principle here laid down, but also arrive at an estimate 

 of the actual limits within which interchange of material has taken 

 place. 



An excellent test-case is afforded by rocks containing calcite. 

 It is well known that impure calcareous rocks are readily meta- 

 morphosed by heat into rocks rich in lime-silicates, with total 

 'Compare Bull. Geol. Soc. Amer. (1891) vol. iii., pp. 16-22. 



