142 ME. ALFRED HARKER OX THE CARR0CK FELL [May 1 895, 



As compared with, the normal granite, the percentage of silica in 

 the greisen shows an increase of about 5, while the figures for mag- 

 nesia, lime, and the alkalies are about halved. Mr. Spencer's 

 analysis shows that some of the mica must be of a soda-bearing 

 variety, and most of the iron can be contained only in the same 

 mineral, accounting for the yellowish tinge already mentioned. 



Quartz-mica rocks with the same general characters as the above 

 occupy the slope down to Grainsgill, and are seen in the beck and 

 again in the lower part of Brandy Gill, encasing the lode which has 

 there been mined for lead. In Grainsgill Beck similar rocks are 

 seen eastward as far as the foot of Poddy Gill, where they are in 

 contact with very highly metamorphosed Skiddaw Slates. West- 

 ward they extend about 250 yards above the junction with Brandy 

 Gill, and re-appear in a small inlier in Grainsgill Beck about 150 

 yards farther. The rock here resembles that analysed. 



On the hill-side south of Grainsgill the greisen is traversed by 

 numerous veins of quartz and mica, from 1 inch to 1 foot in thick- 

 ness, and some few of these also traverse the adjacent metamor- 

 phosed rocks. Quartz forms the bulk of each vein, the grey mica 

 occurring sometimes in patches or pockets, sometimes in distinct 

 seams, where the flakes are often grouped into rosette-like aggreg- 

 ates. Barely the veins contain a few crystals of tourmaline : topaz 

 and tinstone seem to be wholly absent. 



14. Possible Cause of the Modification;, 



It will be seen from the foregoing account that these Cumbrian 

 quartz-mica rocks differ in some respects from typical greisens, 

 such as those of Cornwall and Saxony. On the other hand, they 

 present some points of resemblance to the remarkable rocks in the 

 Urals to which Gustav Rose gave the name beresite. 1 As described 

 by Arzruni, 2 the latter consists essentially of white mica 3 and quartz, 

 sometimes alone, sometimes with more or less orthoclase and plagio- 

 clase. The mica occurs in two habits, and is distinguished by 

 Arzruni as primary and secondary. Other peculiarities of our rocks, 

 such as the porphyritic quartz-grains and the quartzose veins, are 

 also paralleled in the Russian rocks. The typical beresite, how- 

 ever, is richer in mica than the Grainsgill rocks : a non-felspathic 

 example, from Beresowsk itself, yielded only 64*41 per cent, of silica, 

 and was calculated to contain nearly 63 per cent, of mica. For 

 this and other reasons I have preferred the name ' greisen ' to 

 that of ' beresite ' for the rocks here described, while noting their 

 difference from the better known type of greisen. In the beresites 

 of the Urals, as in the rock of Grainsgill, topaz, tinstone, and other 



1 ' Eeise nach dem Ural ' (1837-42), vol. i. p. 186," vol. ii. p. 557. 



2 Zeitschr. Deutsch. geol. Gesellsch. vol. sxxvii. (1885) pp. 865-896. An 

 earlier description by Karpinsky (1875-76), written in .Russian and not very 

 easily accessible, is summarized by Arzruni. 



3 This may include both muscovite and paragonite. In the only chemical 

 analysis of the rock, by Karpinsky, the alkalies are not separately determined. 



