W. M. Hatchings — Claps, Shales, and Slates. 349 



have these quite early and undeveloped stages of the materials of 

 which slates are made, and we see them altered direct into contact- 

 rocks, with development of muscovite, and with a totally new 

 structure. And as they have been left for us in a condition 

 practically not affected by any dynamic action since their alteration, 

 they present us with a particularly good material for study. 



We find that soft shales have become hard compact rocks, and 

 microscopic examination of many of these shows that they consist 

 more or less largely of a criss-cross network of flakes and crystals 

 of regenerated mica, with no relationship whatever to the original 

 plane of the micaceous mineral of the shales. The degree of this 

 regeneration is, of course, variable ; but in the most advanced cases 

 it is very complete. 



The development of chloritic material concurrently with the mica 

 is again well seen. In many cases the mica is largely formed into 

 tufts and radiating sheaves and bundles ; and in these we frequently 

 see that it and the chlorite have crystallized together into such 

 aggregates. 



It is very rare to find biotite formed in these Whin Sill rocks, 

 so that when we are examining the more advanced phases of 

 alteration of very quartz-free shales, we usually have a product 

 made up almost wholly of mica and chloritic material. 



"Spots" are exceedingly abundant in many of these rocks; and 

 though much smaller than at most granite-contacts, they are exactly 

 analogous in all other respects, and vary similarly in colour, etc. 

 Indeed, their nature and mode of origin are more easily made out 

 here than in most contact-areas. Just as we have rocks at granite- 

 contacts which have not formed any spots, so we have them here, 

 the chloritic element being diffused among the mica in all stages, 

 from quite indefinite and almost isotropic matter to more highly- 

 developed chlorite ; while in other cases we have these same 

 varieties of chloritic matter collected into crowds of spots. It is 

 curious to observe how, sometimes, the organic pigment of the 

 shales is concentrated in the spots, leaving the micaceous areas 

 clear of it ; whilst at other times the converse is the case, the spots 

 being free and the pigment remaining in among the muscovite. 



As an instance of these spotted rocks I may mention one from 

 Eowntree Beck, which occurs 18 feet below the Whin Sill. It is 

 mineralogically quite regenerated, no remnant of original mineral 

 being seen ; but it still contains fossils. It was a shale practically 

 free from visible quartz, and is now altered into a mass of white 

 mica and chloritic matter, of which latter a very large part is in the 

 form of almost spherical spots, averaging 200 of an inch in 

 diameter, of dirty yellow-brown colour, into which most of the 

 organic pigment has concentrated. These very abundant spots lie 

 in a field mainly of pure white mica, large portions of this field 

 being relatively clear and colourless in ordinary light, and showing 

 with crossed nicols well-developed flakes and crystals in all 

 directions, and a good many sheaves and spherulitic aggregates. 

 Among this mass is disseminated a good deal of yellow-green 



