20 A.W. Finlayson—Ore-bearing Pegmatites of Carrock Fell. 
Harker has described the greisen which carries the veins,'! while the 
various mineral occurrences have been recorded by Mr. J. G. Goodchild ? 
and others. ; ; 
The Country Rock.—The veins occur in Brandy Gill, a small stream 
running into the Caldew from the western slopes of Carrock Fell. 
The country rock is a detached intrusion of greisen, related to the 
Skiddaw granite, and of post-Silurian age. Mr. Harker has pointed 
out that it is an acid modification of this granite, containing 77:26 
per cent. of silica, as against 75-223 per cent. in the normal biotite 
granite of Skiddaw.* In the ore-bearing portion, felspar becomes very 
subordinate or disappears altogether, white mica replaces biotite, and 
quartz phenocrysts compose the bulk of the rock. ‘Tourmaline occurs 
occasionally, but topaz and cassiterite are absent. In classing this 
rock as a greisen, Mr. Harker has shown that its features are due, not 
in this case to the action of pneumatolytic processes on a normal 
granite, but to intense mechanical pressure on the magma, resulting 
from the post-Silurian crust-movements, at the time of its intrusion.* 
The Grainsgill greisen has many features in common with the 
berestte of the Urals, which contains the gold - quartz veins of 
Berezovsk,® and with the alaskite of Mr. J. K. Spurr.® The latter 
rock, an acid modification of normal granitic or syenitic intrusions, 
occurs as dykes at Goldfield [Nevada], in the Yukon gold-belt of 
Alaska, and elsewhere. These various types are probably closely 
allied in nature and origin. 
The Veins——The wolfram veins in the greisen consist of some 
half-dozen chief members running north and south across Brandy 
Gill, and outcropping on the steep slopes on either side of the valley. 
They vary in width from a few inches to 3 or more feet, and 
consist essentially of coarse quartz, with flakes of pale mica and 
erystals of pale or bluish-green apatite. The vein-walls, though 
fairly well-defined, are not marked by clay selvages or slickensides, 
the veins being, in the phraseology of the American miner, ‘‘ frozen 
to the country.”” Another notable feature is the marked absence of 
pneumatolytic effects, such as the development of schorl-rock, topaz, 
or fluorspar in the vein-zones. 
The most easterly vein, which is also the largest, carries wolframite, 
with a subordinate quantity of associated scheelite, distributed in 
irregular bunches through the vein. ‘The quartz is conspicuously 
banded and comby, and small vughs are common. Some arsenopyrite 
is generally present, scattered through the veins in small crystals. 
Molybdenite also occurs, in its usual habit, but this mineral appears 
to be chiefly developed in a single vein towards the west, which is 
? Quart. Journ. Geol. Soc., 1895, vol. li, pp. 139 et seqq. 
ny 
2 ¢ Contributions towards a list of the minerals in Cumberland and Westmorland ”’: 
Trans. Cumb. and West. Assoc., 1881-2, vil, p. 101; 1882-8, vim, p. 189; 1883-4, 
They |Dn has 
3 Harker, loc. cit. sup. 
4 Id. ib., p. 1438. 
5 Von Arzruni, Zeits. deutsch. geol. Gesell., 1885, xxxvii, p. 878, and Posepny, 
Guide du VII Congrés Géol. Internat., 1897, v, p. 42. 
6 «Geology of the Yukon Gold Belt, Alaska’’; 18th Ann. Rep. U.S. Geol. 
Sury., 1896-7, pt. iii, p. 230. 
