W. M. Hutchings — Rochs of Great Whin Sill. 77 



T alluded, in the paper referred to, to the fact that this substance 

 could be seen in the contact-rocks of basic intrusions, but in less 

 amounts than at granite-contacts. I had not at that time seen the 

 specimen we are now considering, which shows the substance in 

 greater amount than any other I have ever seen, and which 

 strikingly confirms the view at which I had arrived concerning 

 its origin and nature. It here forms a sort of base, or groundmass, 

 all over the slide, and is nearly colourless, there being very little 

 iron present. None of it is quite amorphous, but it has the speckly 

 minutely felsitic polarization. Mica has formed in it throughout, 

 but not regularly diffused, so that whilst at some parts there are 

 patches, large enough to fill the field of. a half-inch objective, in 

 which but a few small distinct flakes are to be seen, we have other 

 portions made up so entii-ely of rnica that little of the base-substance 

 can be seen among it. We can trace the groAvth of the mica in 

 all stages. It is to a large extent in tufts and sheaves and rosettes ; 

 many of these of all sizes, as well as single flakes and crystals, 

 and crossed and interlaced groups of them, maj' be seen brilliantly 

 polarizing, floating free, as it were, in the nearly amorphous material 

 out of which they have grown. 



It is quite clear that what we here see is an intermediate stage, — 

 an interrupted development, — on the road towards some such final 

 product as the two examples we have just been studying. Had 

 the conditions suitable for the crystallization of the mica continued 

 long enough, we should have had a complete and uniform develop- 

 ment of that mineral, together with its attendant chlorite, as before. 

 But it is just the fact that the conditions did not continue long 

 enough for completion of the process, which gives such particular 

 interest and value to this occurrence. Such interrupted cases, when 

 we can get them, are capable of teaching us more of what actually 

 "goes on" in these contact-metamorphisms than any number of 

 completed examples, where often all trace is lost of the steps by 

 which the final result has been arrived at. 



The rutile of the altered shale has crystallized out in much larger 

 and more definite crystals than the original needles, inany of them 

 as "hearts" and "kites," and the entire slide, mica as well as 

 base-substance, swarms with them. A good proportion of the mica, 

 in this case, is brown and strongly dichroic, especially some of its 

 larger tufts and sheaves. There is no clastic quartz remaining ; 

 what little there was evidently entered into the general process of 

 solution, and has reappeared as newly-formed mineral. It is also 

 interesting to notice how the numerous small zircons of the original 

 shale have resisted, as they so frequently do, the solution which has 

 destroyed all trace of everything else, and how they remain quite 

 unaltered among the new products. 



The greater number of shales affected by the "Whin Sill have not, 

 however, been as purely argillaceous as the above examples. They 

 have mainly been more or less sandy. But their alteration has 

 proceeded on much the same lines as those described, and it will not 

 be necessary to consider them in much detail. In some of the more 



