2-8 Geological Society. 



November 18th.— Prof. W. J. Sollas, Sc.D., LL.D., F.R.S., 



President, in the Chair. 



The following commnnication was read : — 



' On some Intrusive Rocks in the Neighbourhood of Eskdale 

 (Cumberland).' By Arthur Richard Dwerryhouse, D.Sc, F.G.S. 



There appear to be five well-marked groups of intrusions in this 

 district : — 



(a) The andesitic dykes in the neighbourhood of Allen Crags and An<de 



Tarn. 



(b) The dj'kes of the spherulitic and felsitic group on Yewbarrow and 



High Fell. 



(c) The dioritic (' bastard granite ') bosses of Peers Grill, Lingmell Crag, 



and Bursting Knotts, with their associated dykes. 



(d) The Eskdale Granite, with the granite-porphyry dyke running from 



Great Bank to Wasdale Head and thence to Kirkfell Crags. 



(e) The dolerite-dykes, having a general north-west to south-east trend. 



The dykes of series (a) bear a very strong penological resemblance 

 to the Borrowdale volcanic rocks, into which they were intruded. 

 Furthermore, they are weathered to much the same extent and 

 have developed the same secondary minerals, among which epidote 

 is conspicuous. They appear to the author to be of Borrowdale 

 age, and roughly contemporaneous with the lavas and ashes into 

 which they are intruded. The spherulitic and more acid series (b) 

 are considered to be also of Borrowdale age, though probably some- 

 what later than the andesitic series. The rocks of the dioritic 

 group (c) are considered to be the noncrystalline and hypabyssal 

 equivalents of the Borrowdale Lavas, and the author is of opinion 

 that they also are of Ordovician age. 



The Eskdale and Wasdale Granites (d) are much more acid, and 

 show little sign of alteration except that due to weathering and 

 dislocation. They are undoubtedly intrusive into the Borrowdale 

 Series, but seem to be pre-Triassic. Thus the intrusion is probably 

 Devonian, like the neighbouring granite of Shap, which, with the 

 exception of its large phenocrysts of orthoclase, is not dissimilar to 

 some of the varieties of the Eskdale Granite. The basic intrusions (e) 

 have been examined ouly where they come into proximity to the 

 granite. They may well be connected with the great Tertiary 

 basic flows of Antrim, as has been suggested by Mr. Harker. 



The granite becomes progressively more and more acid as its 

 margin is approached, until in some places the percentage of silica 

 amounts to 96*16. This is explained by the assumption that the 

 magma as a whole was more acid than the eutectic mixture of 

 quartz '-uid orthoclase, and that consequently the excess of silica 

 separated in the marginal portions, which were the first to solidify. 



