Reports and Proceedings — Geological Society of London. 135 



from the Sengwe Coalfield, by A. Smitli Woodward, LL.D., F.E.S., 

 F.G.S. ; a second, on some Lamellibranoli MoUusca, by Wheelton 

 Hind, M.D., F.R.C.S., F.G.S. ; and a third, on some Fossil Plants 

 from Rhodesia, by E. A. Newell Arber, M.A., F.G.S. 



11. — February 4th, 1903. — Professor Charles Lapworth, LL.D., 

 F.E.S., President, in the Chair. The following communications 

 were read : — 



1. "The Granite and Greisen of CHgga Head (West Cornwall)." 

 By John Brooke Scrivenor, Esq., M.A., F.G.S.^ 



The small granite mass between St. Agnes and Perranporth has 

 been described by Conybeare, Carne, Sedgwick, Foster, and others. 

 It is a remnant of a much larger mass which has been partly 

 denuded by marine action and partly hidden by a north-and-south 

 fault. It is possible to distinguish two divisions of it ; the main 

 mass and the ' tongue,' throughout both of which ' bedding ' is 

 well developed. The granite bordering the bedding-planes has been 

 altered into greisen, which, owing to the abundance of quartz, 

 appears in the cliif-section as dark bands. Each greisen band 

 contains a quartz-vein, marking the original fissure along which 

 metasomatism took place ; the veins contain tourmaline, cassiterite, 

 wolfram, mispickel, and chalcopyrite. Two main reactions appear 

 to have taken place in the formation of the greisen ; the felspars 

 affording topaz, muscovite, and secondary quartz ; the biotite, brown 

 tourmaline, magnetite, and secondary quartz. The fact that no 

 tourmaline has been formed from the felspar, owing to the presence 

 of abundant fluorine, distinguishes this greisen from luxullianite 

 and trowlesworthite. The blue tourmaline prisms included in 

 original quartz appear to have been original constituents of the 

 granite. Secondary quartz, deposited in optical continuity with 

 the original grains, has also caused them to appear to have a crystal 

 outline. The fluorine and boron had not so great an effect on the 

 extremity of the tongue as on the main mass, as shown by the poor 

 development of greisen and the freshness of the biotite. Mica, 

 topaz, and microcline-perthite have been redeposited there by 

 percolating water or vapour. The greisen is an example of Professor 

 Vogt's ' pneumatolytic ' action in thoroughly acid rocks, resulting in 

 the formation of tinstone lodes, as contrasted with the similar action 

 in syenitic rocks with the production of zircon, etc., and in basic 

 rocks with the production of chlor-apatite and the scapolitization of 

 the felspar. 



2. " Notes on the Geology of Patagonia." By John Brooke 

 Scrivenor, Esq., M.A., F.G.S. 



The author was travelling in Patagonia from September, 1900, 

 until March, 1901. The sedimentary strata consist of Tertiary, 

 Cretaceous, and Jurassic formations, which, with the exception of 

 the Jurassic, yield interesting and varied faunas, both vertebrate 

 and invertebrate. The latest classification is that drawn up by 

 Mr. J. B. Hatcher, who conducted the expeditions sent from 



1 Commuuicated by permission of the Director of H.M. Geological Society. 



