TF. M. Kutchings — An Interesting Contad-Roch. 163 



III. — An Interesting Contact-Eock, with Notes on Contact- 

 Metamobphism. 



By "W. Maynard Hutchings, F.G.S. 



[Concluded from the March Number, page 131.) 



THE phenomena to be observed in this special rock seem to me 

 to strikingly bear out some of the ideas and views I expressed 

 in my former paper, when I assumed " that under such conditions, 

 solutions of mineral constituents would be formed of great density, 

 amounting to something 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." 



Of course, it is very largely considered that contact-action is a 

 "hydrothermal business, — that it is due to heated water in some way. 

 But that still leaves the exact mode of operation of the processes 

 very vague, so that it is desirable to follow up any clue, or line 

 of observation which may possibly lead to more definite insight into 

 the course of the changes involved. It is for this reason that I have 

 thought a detailed description of this rock might be of interest. 



Much that was only surmise or inference as to other rocks, may 

 here be seen, as it were, in action before our eyes, and we seem able 

 to follow the whole course of the re-constitution of the components 

 of this shale. Such exceptionally instructive examples will perhaps 

 not often be found, involving as they do a special balance of 

 conditions. 



It will be noted in the analysis that the water is over 7 per cent. 

 This is a high percentage of combined water if compared with 

 that in fully developed hard, compact slates, and with that in their 

 alteration-products. But we are here dealing with what were once 

 comparatively soft shales, which contained more water originally. 

 The water in this analysis is not due to secondary hydrous infiltra- 

 tions. In the large specimens used for analysis care was taken 

 to ascertain, by microscopic examination, that practically no such 

 secondary hydrous minerals were present. The water is mainly 

 contained in some of the minerals of metaraorphism, and in the 

 isotropic base, probably largely in the latter. 



A considerable number of determinations of water have been 

 made in specimens from many points along the contact, and a very 

 uniform amount of about 7 per cent, is found in nearly all the 

 altered shales. This uniformity is striking and agrees also fairly 

 well with the normal contents of water of shales and clays such 

 as these were (see for instance analyses of clays in my former 

 paper). It would appear that the water contained was not di'iven 

 off, and was all utilised in the metamorphism. 



With regard to contact-metamorphism, geologists have tended 

 of late to accept the view that it is due only to thermal action, to the 

 increase of temperature caused by the intrusion of an igneous mass. 

 Indeed, it may be noted that some of our leading writers on the 



