The Granite Area of the Schaioenherg, Somerset West. 201 
lusite ; it is markedly idiomorphic. The tourmaline and the mica occur 
interstitially — enclosing the prisms of andalusite in a similar manner to that 
in which augite encloses the felspar in ophitic structure. The andalusite is 
quite fresh and shows its characteristic pleochroism — in some cases in 
irregular patches ; in others zonally. No quartz was present in the section 
examined. 
(3) Greissen. — The rocks which are here styled greissen occur along the 
north-eastern margin and at the extreme south of the granite area. They 
consist essentially of quartz and muscovite ; some contain a little much 
decomposed felspar. All are characterised in the field by their platy structure. 
The bands vary in width from one, to two or three feet. They are either 
steeply inclined or vertical, and strike E.N.E.-W.S.W. — that is, in the same 
general direction as the quartz -tourmaline veins and the shear planes which 
traverse the area. 
One specimen was obtained composed — except for just a little tourmaline 
in one portion — of light blue-grey quartz. On account of its fine, even- 
grained texture it has the appearance of a quartzite. Inset, however, are 
larger individuals of somewhat darker blue- grey quartz resembling the quartz 
phenocrysts of the granite porphyry. Since the specimen was obtained from 
a small vein-like outcrop — very probably a platy band — the rock might be 
regarded as belonging to this series. 
Quartz-mica rocks, similar to those above described, have been recorded 
— (1) by Mr. A. Harker,* from Orainsgill, in the Carrock Fell district, 
Cumberland ; and (2) from the granite area of Foxdale, I. of Man, by Mr. 
W. C. Simmons. t 
Mr. Harker, in his description of the Grrainsgill greissen, expresses the 
view that the mica is original.:}: Discussing its mode of origin he writes :§ 
" Eegarding it as a modification of the granite, we see that, apart from 
subsequent chemical transformations, it must be due in the first place to 
some process of differentiation. . . . The cause of this differentiation 
was probably that advocated by Mr. Barrow in the case of his pegmatites, 
namely, mechanical force acting on the granite magma when crystallisation 
had already proceeded to a certain stage." The same author refers to the 
granite of Foxdale, I. of Man, as follows :|| "This rock, already intruded 
among Skiddaw slates . . . consists of quartz, microcline, an acid 
plagioclase, muscovite, subordinate biotite, and little crystals of garnet and 
zircon. In the marginal part of the intrusion are developed bands and 
masses of pegmatite, thick veins and bands of greissen, and finally quartz- 
veins containing only local aggregations of white mica or bordered by a 
narrow seam rich in that mineral." 
* ' Quart. Journ. Geol. Soc./ vol. li, No. 202, p. 139. 
t ' Geol. Mag./ No. 566, August, 1911. 
X Loc. ext., p. 141. § Loc. cit., p. 144. || Loc. ext., p. 143. 
