W. M. Hutchings — Clays, Slates, and Contact-Metamorphism. 67 



individual. If this could take place at all it would only be con- 

 ceivable at a temperature which rendered the rock plastic, and even 

 supposing this condition may be attained close to contact we have 

 every evidence that nothing at all approaching to it occurs at the 

 outer zones. We seem to have only the one explanation Avhich 

 harmonises with all we can observe, that this aggregation of the 

 minerals in question is due to solution and recrystallization. We 

 know that no mineral' is absolutely insoluble, even in distilled water, 

 under ordinary conditions. Kutile, e.g., has been used by Dcelter as 

 an example of this fact in his experiments. We know that increase 

 of temperature and pressure, even in the moderate range we can 

 command experimentally, enormously increase the solvent action of 

 water, and that in saline solutions still greater effects can be 

 produced. 



For the observed phenomena of contact action it seems natural to 

 conclude, and probably most people do conclude, that they can be, 

 and have been caused by processes analogous to those in our sealed 

 tubes and digesters. If with the comparatively moderate allowances 

 of pressure, temperature and time granted to us, we can in such 

 apparatus bring about the striking results recorded by experimenters 

 in this dii-ection, what may we not suppose to take place in the 

 enormous sealed tube which is represented by deeply-bedded masses 

 of rocks, highly heated for long periods under intense pressure, 

 with water and solutions which cannot escape. 



It may be assumed that under such conditions solutions of mineral 

 constituents would be formed of great density, amounting to some- 

 thing like an " aqueous fusion " of the substances involved, and that 

 these solutions could solidify to amorphous and more or less indefinite 

 compounds, or would be capable, under some conditions of cooling, 

 of giving rise to definite minerals. 



It seems a not unreasonable supposition that the new substance 

 seen in so many contact-rocks has been formed in this way, and that 

 what we see of it represents various stages of its developments and 

 of its residues ; also that the solution supposed would have a most 

 important part in the transformations of the minerals not originally 

 taken up into it. 



It would naturally be formed in greatest amount nearest the 

 intruding granite, where temperature would be highest and the heat 

 longest continued. There, also, the conditions of subsequent cooling 

 would usually involve greater length of time and greater opportunity 

 for recrystallization, so that in most cases we should have here 

 least of any indefinite residue, or might often have no such residue 

 at all left over for our observation. In the outer zones of the 

 contacts we should have less of the dense solutions, either formed or, 

 more likely, penetrating from the more intensely affected zones where 

 they were abundant ; but as such outer portions would soonest 

 commence to cool again, and cool most rapidly, we should here have 

 a larger proportion of the residues of these solutions left in an 

 undeveloped, or slightly developed, condition. 



Comparatively small portions of such solutions reaching these 



