W. M. Hutchings — An Interesting Contad-Roclc. 165 



rocks. In a very large proportion of these the alkalies, in their 

 quantity in the rock and in their relative proportions to each other, 

 ai"e not so very far removed from wliat obtains in an average granite, 

 and in some cases the resemblance is even striking. If, therefore, 

 a transfer of alkalies took place from the granite, a moderate amount 

 of such transfer would not bring about any change in the chemical 

 constitution of the slates sufficiently striking to serve as proof, 

 especiallj'^ in view of the variations in the slates themselves, which 

 have to be allowed for in making such comparative analyses over 

 certain distances. We know that the alkalies of the granite are just 

 what would chiefly be transferred if a solution passed from it into 

 the slates ; there is ample evidence to show that alkalies were in 

 solution in the liquids of the granites. Therefore, from the above 

 considerations we do not appear to have conclusive evidence, from 

 analyses, that such transfer of solutions did not take place. 



On the other hand, when we come to consider the rocks affected 

 by contact with basic intrusions, we get in many cases very forcible 

 chemical evidence that such transfer did take place. We are here 

 again considering slates and allied rocks, in which the quantity and 

 relationship of the alkalies are the same as before. But in the 

 rocks from which the transfer is supposed to have taken place 

 the alkalies are in totally different proportions. Here we have soda 

 as the leading alkali, and whereas even a considerable transfer from. 

 a granite to a slate would not make a notable difference, such a 

 transfer from a dolerite would often strikingly upset the balance 

 of the alkalies in the slates and become apparent in the analyses. 



Hence it may perhaps be the case, not, as is often assumed and 

 stated, that material is not transferred from a granite but is trans- 

 ferred from a basic rock ; but that it is really transferred in both 

 cases, being only rendered so strongly apparent in the case of basic 

 rocks for the reasons I have suggested. 



Nor can we well imagine any reason which should explain why 

 a transfer should be able to take place from a dolerite and not 

 from a granite. 



If at the time of contact a certain amount of alkaline solution 

 passed into the slates, even though it were not large, it might cause 

 most important effects. It might at least start the whole pi'ocess 

 of transformation, and it may well be conceived as possible that 

 without some such start any mere thermal action would not much 

 affect the mineral constitution of the invaded rocks. The mere 

 water in them might not be able, by itself, to effect the necessary 

 solutions, or only to a limited extent, and with extreme slowness: 

 but once started by the injection of powerfully active solutions at 

 the junction of the rocks, the contained water could join in and 

 carry on the work. Instances might be cited from experimental 

 and industrial work, where analogous influence is exerted by small 

 quantities of some re-agent or compound in thus setting processes 

 going. At any rate there seem to be sufficient grounds for not 

 deciding that contact-action is purely thermal, and we may yet find 

 good reasons for retaining more or less belief in the " mineralizing 



