128 W. M. HutcMngs— Books of Great Whin Sill. 



enough alkali to justify the belief that they could be the early 

 material of true clay-slates, etc. 



It is not, however, in this chemical question, nor yet in the mere 

 cataloguing and description of the new minerals produced, that 

 the great interest of the contact-rocks of the Whin Sill is centred. 

 This interest mainly lies in the observation of the nature of the 

 structures produced, and the relationships of the minerals to one 

 another ; and in the comparison of these structures with those of 

 rocks from great contact-areas round granite intrusions, as well as 

 with those of others, of similar composition, which have been 

 subjected to intense dynamic and deep-thermal conditions, but not, 

 so tar as we know, to the action of any intrusions of igneous masses. 



When a shale is highly altered by contact with the Whin Sill, we 

 get in most cases, as has been shown, a splitting- up of the complex 

 micaceous mineral of which it largely consists, into a purer, more 

 highly developed white mica, which we may in general designate 

 as muscovite (though to some extent it may consist of paragonite 

 or of an intermediate stage), and into a chloritic mineral. Both the 

 mica and the chlorite crystallize in the rock in all directions, quite 

 irrespective of the stratification-plane in which the original micaceous 

 minerals lie flat. A certain criss-cross structure is at once produced ; 

 it commences as soon as the contact-effect is noticeable at all, becomes 

 more and more pronounced in the more highly developed cases, 

 and is often accentuated by the formation of rosettes and sheaves 

 of mica and chlorite. These effects and appearances are all the 

 more striking because they are pi'oduced on an original material 

 of such low development. We pass at one step from a rock with 

 no white mica, no chlorite, and no criss-cross structure, to one in 

 which all these things are in full evidence. At granite-contacts 

 we may often see exactly the same products in the altered rocks, 

 but they are then frequently, in that sense, not so striking; because 

 in most cases the rocks acted upon have been in a much higher 

 stage of development; they were not elementary shales, but slates, 

 in which a good deal of formation of mica and chlorite had already 

 taken place. The final result is, however, the same in both cases, 

 and is also the same whether the granite acts on a mere shale or 

 on a slate ; we get a pure white mica, and a corresponding 

 separation of chlorite or its equivalent in biotite, cordierite, etc., 

 all crystallized in every direction in the I'ock. 



To make out the minerals and the structures in the Whin Sill 

 rocks we may need high powers, whereas we may see all these 

 things with low powers, or a pocket-lens, in the sections from 

 a granite-contact ; but this is a matter of no importance, and is 

 related to nothing beyond the respective bulks of igneous rock 

 concerned, and probably also the greater or lesser depths of the 

 invaded rocks, with corresponding differences of their initial tem- 

 peratures. The same considerations apply also to the frequent 

 abundance of certain special minerals at granite-contacts, and their 

 absence, or rarity, in the Whin Sill rocks. Leaving aside these 

 conditions of mere size and intensity, the results are strikingly 



