410 Geological Society. 



lent no support to the conclusions of Chrustchoff as to the occur- 

 rence of definite types in granite and gneissose rocks respectively. 

 In parts of both the Skiddaw granite and the Threlkeld micro- 

 granite, 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 satis- 

 factorily 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 de- 

 scribed in detail, and it is shown that they are consistently 

 cordierite- and biotite-bearing, like those examined by Mr. Barrow 

 at Home. North of the altered limestone or 'calc-flinta' of Down- 

 town, however, the type of alteration is entirely different, and leads 

 to the inference that the beds are quite distinct. The change is of 

 much more than local significance, as from this point all round the 

 north of the moor, at least as far as South Zeal, there is no bed of 

 any thickness containing cordierite, while chiastolite, white mica, 

 and andalusite proper, are characteristic. Coarse andalusite-rock 

 .and altered shale, with remarkable skeleton-crystals of chiastolite, 

 are described from the Nodden quarries, together with other types 

 of hornfels. It is clear that the beds occupying the northern part 

 of the contact-zone belong to a definite series. There is strong 

 evidence that the cover of the granite mass has a dome-like 

 character, and that precisely the same stratigraphical horizon is in 

 immediate contact with the granite all the way from Sourton to 

 Drewsteignton. 



The granite of Brator, not far from the best exposures of coarse 

 cordierite-hornfels, is described. It is a biotite-bearing rock con- 

 taining a little microcline, as well as orthoclase and oligoclase. It 

 is rich in cordierite, recrystallized from sedimentary 'material 

 absorbed into the magma. Several offshoots from the granite 

 have been noted at various points along the L}~d. One near Noclden 

 -Grate is of interest, as containing topaz in remarkable abundance. 



