70 W. M. Hutching s — Rocks of Great Whin Sill. 



full detail in previous papers in this Magazine. None of the rocks 

 affected had undergone any sort of " development " previous to the 

 intrusion of the Whin ; and as they have not been in any way 

 changed, except by weathering, since the consolidation and cooling 

 of the igneous mass, we are able to see with considerable certainty 

 just what mineralogical and structural changes are to be ascribed to 

 contact-metamorphism. 



Such comparatively simple and reliable conditions, in a contact- 

 area of such importance, are so very rare that it is at once apparent 

 how valuable are the indications we may derive from them, and 

 how great is the assistance they may render to us in our endeavours 

 to understand the much more complex cases usually presented to 

 us. I say " in a contact-area of such importance " because, as 

 I shall show, we have here exactly reproduced for us a large portion 

 of the phenomena we are accustomed to see round the intrusions, 

 on a much mightier scale, of granite, etc. It makes no difference 

 that in greater contact-areas the mineralogical and structural details 

 are more striking as to size, so long as on the smaller scale they are 

 equally clear and distinct. 



I propose to deal with the altered rocks in the following order : 

 pure or almost pure limestones, argillaceous limestones, shales, 

 calcareous shales, sandstones. These, however, pass over into one 

 another in all degrees, and there is, of course, no sharp division 

 between limestones, argillaceous limestones, calcareous shales, 

 shales, quartzy shales, argillaceous sandstones, and sandstones or 

 grits. It is among some of the intermediate rocks that the most 

 interesting effects are produced. 



"When a sufficiently large number of specimens had been sliced 

 and examined, it became evident that there was no use in 

 multiplying them beyond a certain point. It was clear that the 

 same results of alteration could be found at intervals all over the 

 long course of the Whin Sill, wherever the chemical nature of 

 the invaded rocks was the same. For this reason, in the following 

 descriptions, particular localities of occurrence will only be men- 

 tioned when specially interesting or pronounced developments have 

 taken place, which are qualified to serve as good types of the 

 alterations in general. 



Commencing, then, with the limestones, we find that when these 

 are pure, or at all events are non -argillaceous, the action of the 

 Whin Sill upon them has been limited to a recrystallization of them. 

 In some cases this recrystallization is very finely marked, and may 

 stand alongside of the " marmorization " of similarly pure lime- 

 stones by intrusions of granite. 



Whether interfusion has taken place to any extent between the 

 Whin and the purer limestones, at some points, is a question which 

 it is not possible to answer decisively. In many actual contact- 

 slides examined there does not seem to be evidence that any such 

 action has occurred at all ; the division line is quite sharp and clear, 

 the Whin is small-grained but quite crystalline right up to the 

 junction, and the recrystallized limestone begins equally sharply on 



