334 Reports & Proceedings — Geological Society of London. 



EEPOETS -AJSTI P FB OCEEDI^TGS. 



I. — Geological SociETr of London. 

 1. June 9, 1915. — Dr. A. Smith Woodward, E.R.S., President, in 



the Chair. 

 The following communications were read : — 



1. " The Accessory Minerals of the Granitic Rocks of the English 

 Lake District." Ey R. H. Rastall, M.A., F.G.S., and W. H. 

 Wilcockson, B.A. 



The work described in this paper arose out of independent researches 

 on the mineral composition of sediments, in which it became desirable 

 to study the genesis of certain minerals. Preliminary investigations 

 promised results of interest if the rocks of a whole district were 

 examined, and for this purpose the Lake District was selected. The 

 rocks here described are the granites of Skiddaw, Shap, and Eskdale, 

 the microgranite of Threlkeld, and the granophyre of Buttermere and 

 Ennerdale. 



The material was pounded in a mortar, washed and panned, and 

 the concentrate separated in bromoform after the removal of the 

 magnetic portion. 



The general results showed a well-marked variation of accessory 

 minerals between the different intrusions, but a similarity between 

 parts of the same intrusion, although the minerals of apophyses are 

 not always the same as those of the main mass. No evidence is 

 afforded for a genetic connexion between the different intrusions. 



One of the most remarkable results obtained is the rarity of 

 magnetite and the wide prevalence of pyrrhotite which was present 

 in every sample examined, some thirty in number. Special attention 

 was paid to the characteristics of the zircon crystals, which lent no 

 support to the conclusions of Chrustchoff as to the occurrence of 

 definite types in granite and gneissose rocks respectively. In parts 

 of both the Skiddaw granite and the Threlkeld microgranite, anatase 

 and brookite were found in abundance. It was not possible to 

 determine their origin. Epidote is the characteristic mineral of the 

 Ennerdale granophyre, while garnet is abundant at Threlkeld and 

 Eskdale. The Eskdale granite also contains much tourmaline. The 

 Shap granite is especially characterized by apatite and sphene. 



It is concluded that descriptions of accessory minerals founded only 

 on examination of rock-slices are inadequate and misleading; it is 

 only by concentration that the rarer constituents can be satisfactorily 

 determined. 



2. " The Rocks of the Lyd Valley, above Lydford (Dartmoor)." 

 By Frederic Philip Mennell, F.G.S. 



The paper deals chiefly with a small area on the north-east of 

 Dartmoor, though some of the conclusions are applicable to nearly 

 all that part of the moor which lies north of the portion recently 

 mapped by the Geological Survey. In the immediate neighbourhood 

 of Lydford the progressive alteration of the Carboniferous rocks 

 within the metamorphic aureole surrounding the granite is described 

 in detail, and it is shown that they are consistently cordierite- and 

 biotite-bearing, like those examined by Mr. Barrow at Holne. North 



