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W. M. Hutchings — Rocks of Great Whin Sill. 123 



VII. — The Contact-Rocks of thb Great Whin Sill. 



By W. Maynaud Hutchings, F.G.S. 



{Concluded from the February Number, page 82.) 



N describing " An Interesting Contact-Eock" (Geol. Mag., March 

 and April, 1895) I gave its analysis, showing; — 



Potash ,., ... ... ... ... ... 1-25 per cent. 



Soda 2-01 ,, 



And in this rock also the great relative increase in soda coiTesponds 

 with the appearance of alhite among the new minerfils. 



But, as stated above, we also have cases of great alteration without 

 increase of soda ; as, for instance, the two intensely affected shales 

 described from Rowntree Beck and Winch's Bridge, both very rich 

 in alkali, and both showing still a normal excess of potash. And, 

 again, three other specimens of completely altered rocks from near 

 contact give : — 



Potash 3-lo per cent. ... 4'24 per cent. ... 3'40 per cent. 



Soda 1-20 ,, ... 1-22 ,, ... 0-87 



In these the mica is all regenerated, with chlorite, as described, but 

 there is no sign of any new felspar. 



Without burdening this paper with too many figures, I may say 

 that in between these extremes of great alteration of alkali-ratio, 

 and no alteration at all, come determinations giving an intennediale 

 result — soda has increased beyond the normal limits, but still not 

 to the extent of exceeding the potash, as for example : — 



Potash 3-55 percent. ... 1*60 percent. 



Soda 2-20 ,, ... 1-31 ,, 



If soda-transfer takes place, we should expect to find evidence 

 of it in the less pure sandstones, as well as in the shales, those with 

 argillaceous interstitial matter being really only diluted shales. 

 ] have determined the alkalies in three specimens of altered sand- 

 stones, as follows : — 



Potash ... 0-25 per cent. ... 0-55 per cent. ... 0-23 per cent. 



Soda ... 2-46 „ ... 2-71 ,, ... M^ ,, 



Here there is in each case a very decided excess of soda, and 

 the tot<il alkali contained is much more than would be expected in 

 consideration of the relatively small amount of original argillaceous 

 material. These determinations, as far as they go, would strongly 

 countenance the idea of transfer of soda in some form, and tend 

 also to show that the sandstones have taken it into combination, 

 and held it, out of proportion to the shaly deposit contained in them. 

 Tests of some of the limestones have also been made, for the 

 same reason, that where less pure they contain argillaceous material, 

 and should show the alteration of alkali-proportion if it has taken 

 place. Of the following two analyses, A represents a specimen from 

 within a few inches of contact, containing a quantity of garnet and 



