404 Arthur Holmes — The Granophyres of Carroclc Fell. 



therefore more accurately described as albite quartz-porphyry. 

 Mr. Hemingway states that the rock was first noted in some mine 

 workings near the head of Brandy Gill, where it was covered at the 

 surface by a white decomposition product. The latter, which is 

 known as "the china-clay bed" has been traced for more than 

 a mile in a westerly direction. At Arye-stones the deposit is very 

 extensive, but the fresh rock below was cut through by the old 

 levels of the Roughten Gill lead-mines. At Brandy Gill the rock is 

 penetrated by the tungsten veins of the Carrock mines, which carry 

 scheelite and wulfenite. 1 



Petrography of the Albite Granophyre. 



The granophyre of the Carrock Fell complex and its variations 

 are well known from the description by Dr. Harker. 2 He describes 

 the normal granophyre as showing 3 "small scattered crystals of 

 black augite [diopside approaching hedenbergite] and white or 

 glassy-looking oligoclase, in a fine-textured grey or cream-coloured 

 or reddish groundmass". He then continues, "In some of the more 

 acid examples the augite is wholly or almost wholly absent, and the 

 rock has a white colour. This is the case at the head of Brandy 

 Gill and in the peat moss south of Drygill Head, and the specific 

 gravity of these specimens is naturally very low (2*578 and 2"530)." 



The specimen analysed by Dr. Harwood. is a white quartzite-like 

 rock with sparsely distributed rectangular crystals of felspar which 

 average about 3 mm. by 1 mm. in areal dimensions. The specific 

 gravity is 2*63. 



Under the microscope the felspar phenocrysts are seen to be dusky 

 from incipient alteration, and to carry as inclusions small films and 

 fans of muscovite. Albite, pericline, and carlsbad types of twinning 

 are developed. A few cleavage flakes were obtained by crushing 

 the rock, and the extinction angles (up to 19° on 010), mean refractive 

 index (between V53, chlorobenzene, and 1*54, clove oil), and specific 

 gravity (2'63 in a Klein solution of that density) indicate that the 

 felspar is albite, a conclusion which is verified by the analysis. 



The rock is crowded with small irregular crystals of quartz, 

 often corroded, and varying regularly in size from undoubted 

 phenocrysts to small masses that form part of the micrographic 

 groundmass. The latter is of the finely textured type described by 

 Harker, 4 with occasional felspar nuclei in optical continuity with that 

 of the surrounding intergrowth. There are, however, numerous 

 minute wisps of muscovite in the groundmass, and much of the 

 felspar of the latter is altered to cloudy aggregates of sericite. 

 Moreover, the rock contains here and there small fans of radiating 

 muscovite with which zircon in well-crystallized prisms or rounded 

 grains is invariably associated. No definite pyroxene has been 



1 For an account of similar veins in the Grainsgill greisen see A. M. 

 Finlayson, Geol. Mag., Dec. V, Vol. VII, p. 19, 1910. 



2 "The Carrock Fell Granophyre": Q.J.G.S., li, p. 125, 1S95 (Map of 

 the Carrock Fell District, plate iv). 



3 Ibid., p. 131. 



4 Loc. cit., p. 128. 



