The Metallogeny of the British Isles. 821 



is not here dealt with. The normal granite is more or less por- 

 phyrilic in structure, with large phenocrysls of perthite, in a coarse 

 or tine-textured ground-mass of orthoclase, plagioclase, biotite and 

 muscovite. Various microgiaphic intergrowths occur, indicating 

 in some cases eutectics of three or more components. No reliable 

 analyses are available, but the rock is provisionally classed as an 

 alkali-granite of Hatch's classification. 



Evidence is brought forward to indicate that the granite is 

 intruded along the axis of an anticline, Avith a strike approximately 

 E. 15° N. and W. 15° S., the normal direction for the district. 



The metamorphic aureole is very large, measuring about 6 miles 

 from east to west, and 5 miles from north to south. This is out 

 of all proportion to the size of the visible exposures of granite, 

 and it is inferred that the intrusion underlies a large area at a 

 small depth. 



Within this area three distinct rock-types can be recognized : 

 namely: (1) black slates; (2) grey flags; (3) grey grits. The 

 mefamorphism produced in each of these is described in detail, and 

 it is shown that the commonly accepted zones of alteration do not 

 hold, since the rocks concerned were originally of very different 

 character. The frequently described section in the Glenderateira 

 valley runs across the strike, and includes both black slates and grey 

 flags. The former never undergo a high degree of metamorphism, 

 chiastolite being the characteristic mineral. The well-known 

 cordierite-mica rocks of Sinen Gill are derived from the grey flags, 

 and a very narrow zone close to the granite shows garnet and 

 staurolite. The impure grey grits of the central band contain 

 cordierite, andalu>ite. and mica, and garnets are only *e^n close to 

 the contact with the Grainsgill greisen. The Carrock Pell intru- 

 sion produces little or no alteration in the grits, with which it 

 comes into contact for a long distance. 



The phenomena here displayed may be summed up as an example 

 of a moderate degree of thermal metamorphism, due to the intrusion 

 of a large mass of granite, at a comparatively low temperal ure, into 

 a series of rocks of variable composition, which had previously 

 undergone dynamic metamorphism. The most important minerals 

 produced are cordierite, andalusite and chiastolite, biotite and 

 muscovite, while garnet and staurolite are only found (dose to the 

 granite. Owing to the variations of lithological composition across 

 the strike, it has not been found practicable to divide the aureole into 

 concentric zones, but the alteration is gradual and progres.-dve 

 towards the intrusion. 



2. 'The Metallogeny of the British Isles.' 13v Alexander Mon- 

 crieff Finlayson, M.Sc.', A.O.S.M., F.G.S. 



The ore-deposits of the British Isles (tin. copper, lead, zinc, 

 gold) are considered synthetically in their relation to igneous rocks 

 and to tectonics. The four major epochs of igneous activity and 

 crust-movement in the area were: pre-Cambrian. post-Silurian 

 (Caledonian), post-Carboniferous (Hcrcynian), and Tertiary. A few 



