A. M. Finlayson—Ore-bearing Pegmatites of Carrock Fell, 23 
is very frequently due to metasomatic replacement, and this may take 
lace in one of two directions. There may be a replacement of the 
acid radicle by tungstic acid; or there may be a replacement of the 
base by lime, or by iron and manganese oxides. The former process 
has taken place, for example, in the scheelite deposits of Trumbull, 
Connecticut,’ and in the scheelite-veins of Otago, New Zealand.? In 
both these cases, tungstic acid has combined by replacement with 
lime-bearing minerals in the vein-zone. Another important example 
is the wolfram-ore of the Black Hills, South Dakota. The second 
process is observed where scheelite is metasomatic after wolframite, as 
in the present instance; and likewise where the reverse change has 
occurred. ‘Thus, in the deposits at Trumbull, Connecticut, wolframite 
occurs pseudomorphous after scheelite. The latter change, i.e. from 
scheelite to wolframite, appears to be much the commoner. 
Fissure -veins at Grainsgill. —There occur, associated with the 
Grainsgill pegmatites, typical lead-quartz veins in fissures. An 
example is to be seen withina few yards of the most westerly 
pegmatite-vein, and contained also in the ereisen. It is bounded by 
sharply defined walls with clay selvages. The vein-filling is of 
brecciated, or banded and cavernous quartz, and the ore consists of 
small bunches of well-crystallized galena, with a little chalcopyrite. 
This vein has clearly been deposited in a fissure formed in the rock 
long subsequent to its consolidation, the fissure being, like all those 
which carry the lead-zine and copper veins of the Lake District, of 
post-Carboniferous age. We have here, then, two distinct vein- 
types, which show a striking difference, both in origin and in age. 
In this connexion, it is worthy of note that the lead-veins of the 
Lake District have yielded two minerals allied to those in the 
pegmatites, namely, wulfenite from Caldbeck Fell* and stolzite from 
Force Crag.° There has apparently been a reaction, at considerable 
depth, between the older tungsten and molybdenum ores and 
the younger lead-ores, which points to a broad genetic connexion 
between them. 
Genetic Relations of the Grainsgill Pegmatites—The modern views 
on the nature and origin of pegmatites have been recently summarized 
by J. B. Hastings,® who concludes that their essential feature is their 
dependence on aqueo-igneous intrusion, as applied to the influence of 
water in conjunction with heat in causing the liquidity of granitic 
magmas. In different cases it may be difficult to determine to what 
extent igneous intrusion on the one hand, and magmatic waters on the 
other, have been predominant, and it is impossible to postulate 
exclusively either aqueous or igneous agency, since there has clearly 
been a combination of both factors. G. H. Williams, in discussing 
the origin of the Maryland pegmatites,’ concludes that they are of 
1 W. H. Hobbs, 22nd Ann. Rep. U.S. Geol. Surv., 1900-1, pt. ii, p. 18. 
ae M. Finlayson, ‘‘ The Scheelite of Otago’’: Trans. N.Z. Inst., 1907, xl, 
. (. 
r 3 J. D. Irving, Trans. Amer. Inst. Min! Eng., 1902, xxxi, p. 683. 
4 J. G. Goodchild, Grou. Mac., N.S., 1875, Dec. V, Vol. IT, p. 565. 
° Greg & Lettsom, Mineralogy of Great Britain, London, 1858, p. 411. 
6 Bull. No. 21, Amer. Inst. Min. Eng., May, 1908, p. 319. 
7 15th Ann. Rep. U.S. Geol. Surv., 1893-4, p. 678. 
