130 W. M. Hittchwgs—RocU of Great Whin Sill. 



far as our present actual knoioledge goes, there are structures and 

 raineralogical developments which, whenever we see them, even 

 in moderate degrees of evolution, we are not only justified in. 

 ascribing to contact-metamorphism, but which we have not an atom 

 of reason or evidence for attributing to any other cause, no other 

 cause having as yet ever been proved to produce them; and certainly 

 not dynamic action, as to which we can collect plenty of very clear 

 evidence that it has failed over and over again to bring about even 

 the beginnings of them, under the very circumstances which ought 

 to be most favourable for its doing so. 



When we" look at this matter quite calmly, it certainly does seem 

 rather strange that the " blessed word " dynamometamorphism has 

 been allowed to exercise such a spell over our minds in directions 

 in which it can hardly be said to have ever made good its pretensions. 

 Here were rocks of sedimentary origin, showing very great and 

 striking mineralogical developments. They also showed beyond 

 question that they had undergone great dynamic action. Therefore 

 the latter was the cause of the former. In how many cases 

 has there been but little better evidence than this to support its 

 all-embracing claims, which were made to explain everything 

 without proper proof! And at the same time we have, all around 

 us, examples of the fact that what dynamic action has been asserted, 

 but not proved, to do, is done not only by every great intrusion of 

 granite or other igneous rock, but by even quite small intrusions also. 



Let a great area of sedimentary rocks be altered by the action 

 of igneous masses which we cannot see ; then let powerful dynamic 

 action follow, and there we have at once a fine example of the 

 marvellous recry stall ization and formations of new minerals which 

 dynamic metamorphism has brought about, shutting our eyes to 

 other cases in which even more intense action, on similar materials, 

 has effected practically nothing of the sort. 



If we take simply what we at present know, and can prove over 

 and over again, and separate it from what is certainly not at all 

 proved, no matter how strong the a priori evidence may sometimes 

 appear, it would seem to be quite reasonable if we were to regard 

 certain microscopic structures of altered sedimentary rocks as 

 probably indicating that the alteration took place under the influence 

 of contact-metamorphism, no matter whether we can actually see 

 the igneous rock concerned in it or not. On a similar line of 

 reasoning from what we know, we might also draw the same 

 inference from the development of certain minerals in such rocks, 

 not only the specially so-called "contact-minerals," but others as 

 well. Thus, the presence of undoubtedly newly-formed biotite in 

 altered shales and slates should point the same way, till we have 

 some evidence that any other process known to us can be proved 

 to have the power of producing it. And it might even not be 

 going outside the safe ground of induction if we were to include 

 very highly developed and individualized muscovite, and certain 

 •*"<)rm8 of chlorite, under the same head. 



■^f these several points, that of structure certainlj'^ appears to be 



