68 W. M. Hidchings — Clays, Slates, and Contact- MetamorpMsm. 



outer regions would tend to draw together, as little spots and 

 patches, among the ingredients of the otherwise not much affected 

 rock, and would consolidate in this form. 



As in contact-regions generally there can frequently be observed 

 anomalous phenomena, such as strata, or portions of strata, which 

 have been much less or much more affected than others close to 

 them, without our being able to explain the fact, so we have 

 anomalies in the behaviour of these supposed dense solutions, and an 

 inner zone may sometimes show a large amount of the less developed 

 residual matter, as in the case of the hornfels mentioned above from 

 Spitzenberg, and other cases of which I have specimens. A notably 

 good example occurs in one of a series of rocks from the Elbe Valley, 

 in which the large amount of the substance present surrounds and 

 envelops the minerals in such a manner that a glance at a section 

 is enough to convince the observer that it was present as a fluid 

 medium when these minerals crystallized. 



If we look at all the changes which have taken place in a well 

 developed contact rock, like that at Shap for instance, it seems 

 absolutely necessary, in order to explain what we see, to assume that 

 almost all the original ingredients have been taken up into some 

 sort of solution and completely newly formed out of it. It is not 

 necessary to assume that they were at any one time all in solution 

 at once, or even a very large portion of them. 



A solution, or solutions, would be formed of those ingredients of 

 the rock which were most readily acted upon by the imprisoned 

 liquids, and the perhaps only relatively small amount of solution 

 so formed would act as a medium for the slow solution and 

 recrystallization of minerals, just as relatively small amounts of 

 certain solutions and fluxes have been made to act in recorded 

 experiments. The composition of the first solution would not be 

 always the same. It would vary with the varying composition of 

 the slates, etc., acted upon, but probably not in very wide limits, as the 

 qualitative composition of most slates is strikingly uniform. The 

 variation might, however, be expected to be sufficient to influence 

 the products which would result from the crystalline development 

 of the solutions. According to my observations the most usual 

 development gives rise to mica in greatest quantity, but felspar 

 and quartz also result in some cases. 



Let us, finally, glance at the principal mineral changes which we 

 know to take place in contact-metamorphism, and see what con- 

 clusions we seem justified in drawing as to how they have come 

 about, and how these harmonise with the ideas above suggested. 



(1) Rutile. — By universal testimony we find that in all contact- 

 areas of slates, etc. (whether the intrusive rock is granite or a more 

 basic rock) one of the most unfailing, as well as most sensitive, in- 

 dications of commencing metamorphism is that the minute "clay slate 

 needles " undergo an alteration by which they decrease in number 

 and increase in size, at the same time that they become blunter in 

 proportion to length and have more definite pyramidal terminations. 

 This occurs so far from the intruding rock that we cannot suppose 



