W. M. Hutching s— Rocks of Great Whin Sill. 129 



parallel ; anrl a concurrent examination of a series of rocks from 

 the two sources does not fail to impress one with the idea of how 

 relatively slight and mild a degree of contact-action is required to 

 bring about certain very definite and characteristic effects. I have 

 compared these Whin Sill rocks over and over again with contact- 

 rocks from several localities, and quite recently, whilst once more 

 passing them in review in connection with the putting together of 

 these notes, I have had the opportunit}' of looking at them side by 

 side with a fine series from Scotland. One can often pick out 

 examples of both, which, barring certain minerals (as e.g. kyanite), 

 are so exactly similar in structure and general nature, that the slide 

 from the granite-contact might be almost imagined to be derived 

 from one from the Whin-contact by some process akin to photo- 

 graphic "enlargement," every detail being reproduced. 



But, on the other hand, we may compare these Whin Sill sections 

 with any number of examples of rocks which have suffered the most 

 intense degrees of crushing and shearing, and which have been 

 under enormous depths of cover, without being able to find in 

 these latter any signs of even a commencement of the characteristic 

 structtires, or more than a very moderate amount of the mineralogical 

 development. Such rocks as these, as I have shown in former 

 notes (e.g. Geol. Mag., July and August, 1896), certainly display 

 a decided degree of advance beyond the clays and shales from which 

 they started. We have the formation of new mica and of chlorite, 

 going along the same chemical and mineralogical lines as in contact- 

 action ; but it does not seem to be able to pass beyond relatively 

 moderate limits, giving us a still impure mica. And in the matter 

 of structure the limits are still more restricted. We do not get 

 beyond a felte<l and wavy mass of mixed mica and chlorite ; there 

 is no growth of crystals of mica at all angles and in all directions, 

 no criss-cross structure, no rosettes and sheaves, and no sign any- 

 where of crystallization of chlorite. Neither do we ever see any trace 

 of the amorphous, or semi-amorphous, and speckly material passing 

 upwards into definite mica, etc., which I have pointed out as so 

 frequently characteristic of rocks which have recrystallized under 

 contact-action. Nor do we see in such rocks any trace of biotite 

 or other special minerals which we know so well in contact-rocks. 

 Yet all these things which we may thus find to be absent from some 

 most ancient sedimentary rocks after they have had every allowance 

 of time, dynamic action, and depth-conditions, we see can be pro- 

 duced in similar materials, almost instantly, as it were, by the 

 action of a relatively insignificant amount of igneous magma 

 intruded among them ; the evidence in the special case before us 

 being quite beyond the possibility of confusion or question, as to 

 the fact that these effects are wholly due to the intrusion and to 

 nothing else, and thus much simpler and clearer than often is the 

 case in granite areas. 



These observed facts, and the considerations arising out of them, 

 duly weiglied, seem to lend a reasonable degree of probability to 

 the conclusion I have suggested on other occasions, viz., that so 



DECADE IV. — VOL. V. — NO. III. 9 



