Geoloi Ileal Socii'tji. 541 



case of bodies whose tbermal state requires more than two 

 variables for its complete specitication. 



The style is admirably clear, and tlie book will prove particularly 

 useful to advanced students both on account of its comprehen- 

 siveness and its self-contained character, which renders unnecessary 

 frequent and troublesome references to textbooks on allied subjects. 



LXYIII. Proceedings of Learned Societies. 



GEOLOGICAL SOCIETY. 



[Continued from p. 884.] 



February 4th, 1903.— Prof. Charles Lapworth, LL.D., F.ll.S., 

 President, in the Chair. 



''PHE following communications were read: — 



-*- 1. ' The Granite and Greisenof Cligga Head (West Cornwall).' 



By John Prooke Scrivenor, Esq., M.A., E.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 cliff-section as dark bands. Each greisen-baud 

 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 wdth 

 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 re-deposited there by per- 

 colating water or vapour. The greisen is an example of Prof. Togt'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. 



