W. M. Hutchings — Rochs of Great Whin Sill. 127 



unless it can be shown that there are shales, and argillaceous sand- 

 stones and limestones, among the Lower Carboniferous beds, and 

 indeed in the particular districts in question, containing soda in 

 some such ratio to potash as is disclosed by these determinations. 

 On the other hand, it is difficult to explain the fact that such soda- 

 rich alteration-products alternate with others, derived apparently 

 from quite similar original rocks, in which, as we saw, soda has 

 not increased, or has increased in a far less degree. 



Supposing some compound of soda to pass from the igneous 

 magma into the invaded beds, we can readily explain to ourselves 

 how it could come about that purer limestones show little or no 

 trace of its action. Solutions containing this compound of soda 

 could permeate the limestone, and pass into beds of shale, etc., 

 beyond it, without being in any way permanently taken up and 

 combined with it ; and in course of time the limestone would be 

 freed from the merely mechanically held soda; whilst in the 

 complex silicates of the shales would be offered a material with 

 which introduced soda would easily enter into new and permanent 

 combinations. But we cannot very well explain how it is that one 

 bed, or part of a bed, of shale takes up so much more than another. 

 We must leave this, for the present, as one of the many things 

 which we cannot yet make clear. 



Looking at those rocks which show a considerable amount of 

 soda- felspars, and carefully observing the part these felspars play 

 in the structure of the rock, and their relationships to the other 

 minerals, it does not seem possible to conclude otherwise than that 

 the soda-increase and formation of these felspars and these structures 

 were all part of the one process of contact-metamorphism and 

 recrystallization of the rock-constituents. No later introduction of 

 soda, by percolation of solutions from the cooled and weathering 

 igneous rock, is at all consonant with what we see. x\nd this is 

 equally true of those beds in which, though we find soda in excess, 

 we cannot detect any felspar with the microscope, and are led to 

 assume that the soda is combined in the abundant new mica 

 developed, in the " speckly " intermediate material, and in other 

 new products. 



In this rather indefinite and unsatisfactory position we must, 

 apparently, leave this part of the subject for the present, at all 

 events so far as the Whin Sill is concerned. Having devoted 

 a good deal of time and trouble, both to " looking it up " elsewhere, 

 and to trying to obtain evidence from our most favourable British 

 opportunity of studying it, I make no apology for dealing with it 

 at some length, even though, unfortunately, not conclusively. It 

 touches one of the most important points which still stand first for 

 consideration on our way to an understanding of the true nature and 

 processes of contact-metamorphism. 



Incidentally, it is worth pointing out that the series of alkali- 

 determinations given above, supplementing as they do my previous 

 analyses of fireclays and shales, quite definitely dispose of the 

 contention, often put forward, that none of these deposits contain 



