74 W. M. Hutchings— Rocks of Great Whin Sill 



It is not only close to contact that these limestones have been 

 aflfected. Complete recrystallization is seen at more than 60 feet 

 away, and small augite crystals are seen in a specimen over 40 feet 

 distant. 



We may now turn to the consideration of the alteration-products 

 of the shales. These shales along the contact-area of the Whin 

 Sill have varied in nature in every degree, from argillaceous beds 

 almost quite free from quartz, to others in which that mineral has 

 formed a large proportion. For our present purpose we will call 

 them all shales, speaking of them as more or less sandy, and only 

 draw the line where the quartz has increased so largely that we 

 must recognize them as argillaceous sandstones and classify them 

 accordingly. Of this class of rock a very large number of 

 specimens have been examined, but here again a few examples will 

 suffice to give a clear idea of the genei'al lines on which the 

 metamorphism has proceeded. The intensity, and to some extent 

 also the character, of the alteration varies more or less at different 

 places. This is partly due to different composition, notably the 

 variation in the amounts of quartz and of alkalies contained affecting 

 the susceptibility of the beds to contact-action. Partly it is due 

 also to the varying bulk of the intruded rock at different points, and 

 sometimes we cannot account for the variation except by assuming 

 some difference in the local conditions of the invaded beds, as to 

 temperature, degree of hydration, etc., before the intrusion occurred. 



Outwardly the change undergone by the originally soft shales 

 consists in great induration, accompanied by more or less lightening 

 of colour. In the inner zones of action, nearer the igneous rock, 

 (sometimes also at considerable distances), the soft, fissile, dark- 

 coloured shale has been altered into a hard, compact, grey or 

 greenish-grey rock, with often very little fissility remaining, and in 

 many cases completely replaced by a conchoidal or almost flinty 

 fracture. 



Inwardly, as revealed in thin sections, the most constant and 

 striking change lies in the production of new mica, with chlorite, 

 with a totally new structure as well as new mineralogical com- 

 position. Newly crystallized quartz is also frequently a main 

 feature, in some cases felspar has been abundantly produced, and we 

 have examples of the appearance of special contact-minerals in the 

 form of bii)tite. andalusite, anthophyllite, etc. 



If we take first the purest shales, which have had little, if any, 

 quartz, and which have a chemical composition like that of some 

 of the purest " fireclays," we find that where the contact-action has 

 been most intense we have a complete recrystallization of the entire 

 rock, with formation of a new mass of white mica throughout. 

 Instead of the minute flakes of the indefinite micaceous mineral 

 which has been produced in the fireclay or shale, making its main 

 constituent, and lying nearly all in one plane, we get a network 

 of much larger, well-developed flakes and crystals of white mica, 

 lying criss-cross in all directions. In many cases a good deal of it 

 is grouped together in fans and sheaves, and roughly spherulitio 



