SECTIONAL TRANSACTIONS.— C. 327 



Granite near rich lode. 



Much muscovite, chlorite, and tourmaline. 

 Felspars much sericitised or kaolinised. 



Granite near low-grade lode. 



Some biotite, little tourmaline, felspars only slightly altered. 



A limited number of copies of a paper giving detailed results are available for those 

 specially interested. 



Mr. E. H. Davison. — The Cornish Pegmatites. 



Pegmatites occur in Cornwall both as veins in the granite and also as veins pene- 

 trating the overlying altered slates. When in the granite they are most common near 

 the margin of that rock, but are also found well inside the granite mass, an instance 

 being the pegmatite cut in Williams' shaft, Dolcoath Mine, which was cut at a depth of 

 nearly 4,000 feet from the surface of the granite. 



They vary much in coarseness of texture, some being exceptionally coarse with 

 felspar crystals up to 18 inches in length and 4 inches square in cross-section, while 

 others are little coarser than the normal Cornish granite. 



In composition they also vary considerably. Quartz and orthoclase are invariable 

 components, and a mica is usually present which may be muscovite, zinnwaldite, 

 or gilbertite, biotite being rare. Tourmaline is a common constituent, while fluorite, 

 apatite, pinite, cassiterite, wolfram, and mispickel also occur. 



One of the characteristic features of the Cornish Pegmatites is the occurrence of a 

 banded structure, the veins being built up of alternating bands of pegmatite and aplite 

 or having a centre of pegmatite with selvages of aplite or of a much finer grained pegma- 

 tite. One dyke of pegmatite (at Trelavour Downs, St. Dennis) shows exceptional 

 banded texture, the margins consisting of medium-grained brown (lithia-bearing) 

 mica and felspar, followed by massive fine mica and then massive coarsely crystallised 

 mica with a centre of coarse mica, felspar and quartz. 



Another feature is the maimer in which veins of pegmatite hi the slate shade off 

 into aplite and in some cases into quartz veins. This can be seen at Porthmeor 

 Cove, Gurnard's Head, and was observed in the Roskear shaft, where veins of pegmatite 

 were seen to pass into aplite, and aplite into quartz veins. 



That the temperature was low at the time of solidification of the pegmatite is 

 shown by the fact that pegmatite veins in the slate produce very little alteration in 

 that rock at the contact. At Megilligar Rocks, near Porthleven, the largest vein of 

 pegmatite, some 10 feet wide, alters the slate for a distance of only an inch or two 

 from the margin, while the vein contains slate xenoliths of small and large size which 

 show only an alteration selvage of about half an inch. 



The close relation between the intrusion of pegmatite veins and the formation of 

 mineral lodes is also shown by the occurrence of metallic minerals such as cassiterite, 

 wolframite, mispickel, &c, in the veins, and this is supported by the occurrence of 

 mineral lodes with pegmatite-like structure and containing patches of granitic material 

 at points well outside the granite itself. 



Afternoon. 

 Prof. W. G. Fearnsides. — Report of the Dolgarrog Committee. (See p. 276.) 

 Excursion to Ackton Hall Colliery. 



Tuesday, September 6. 



Joint Discussion with Sections A (Cosmical Physics Dept.) and K 

 on The Climates of the Past. (See p. 386.) 



Mr. H. C. Versey. — Post-Carboniferous Movements in the Northumbrian 

 Fault Block. 



By reason of its rigidity this area was unable to fold, and in consequence a largo 

 system of fractures originated, and the area was uplifted as a fault block. The 



