Page:The Journal of geology (IA journalofgeology11893univers).pdf/149

Metasomatic changes are known to take place during thermal metamorphism as regard 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 invading igneous magma to the neighboring rocks, but such a phenomenon seems to be of uncommon occurrence, and to be confined 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 transference of material (other than volatilve substances) is confined to extremely narrow limits, and consequently that, for a given temperature 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. Illustrations of this principle, as stated in the latter form, are familiar to all who have studied cases of "contact metamorphism":^[1] 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 metamorphosed by heat into rocks rich in lime-silicates, with total