I 6-52 



W. M. Htdchings — Rochs of Or eat Whin Sill. 81 



Another specimen of a like nature, from close to contact, near 

 to Crag Lough, Bardon Mill, has the following composition : — 



Silica 



Aluiniua ... 



Ferric Oxide ... 



Lime 



Magnesia ... 



Potash 



Soda 



Water and Carbonic Acid 



100-82 



It resembles the others in general composition, but shows hornblende 

 among its new minerals. 



It now remains to consider the sandstones, of which a large 

 number have been collected from different points. There is not 

 very much to be said about them, because when they are pure, or 

 nearly so, the alteration is limited to a compacting and conversion 

 of them more or less into quai'tzites ; and where they are less pure, 

 the interstitial matter has undergone the same alterations as have 

 been above described. Thus, where there has been any noticeable 

 amount of argillaceous deposit with the quartz-grains, it is now 

 often seen to consist largely of the same mixture of new white mica 

 and chloritic matter ; and in this way we pass back again towards 

 altered sandy shales, as the interstitial constituents increase. 



It is noticeable, however, that whereas among the altered shales 

 it is but seldom that brown mica is seen as a contact-mineral, and 

 then only to a very subordinate degree, we find it more frequently 

 and much more plentifully among the argillaceous sandstones. In 

 one case from Kumbling Churn, near Dunstanburgh, there is as 

 large a development of this biotite as might occur at any granite- 

 contact, and all the characteristics of the mineral are the same. 



In previous allusions to the contact rocks of the Whin Sill 

 (Geol. Mag., April, 1895) 1 had occasion to refer to the interesting 

 question of the supposed transfer of soda from the igneous rock to 

 the altered shales, etc., in such cases of basic intrusions. I pointed 

 out that observers of the contact-effects of such rocks elsewhere had 

 been forced to come to the conclusion that such a transfer does often 

 take place, a very considerable mass of chemical and other evidence 

 rendering any other verdict difficult, or even impossible. Most of 

 our knowledge on this point comes to us from German petrologists, 

 though instances of altered rocks rich in soda are not lacking in this 

 count r3^ 



When I commenced working at the petrology of the Whin Sill 

 contact I naturally gave attention to this very important point, 

 and it so happened that some of my first analyses, and separate 

 determinations of alkalies in altered shales, very strongly confirmed 

 the views expressed by the German authorities. In the course of 

 the work I have made a considerable number of further determina- 

 tions, the general result being that the answer obtained is not at all 

 uniform in its direction. There are many of the shales in which soda 



DECADE IV. VOL. V. NO. II. 6 



