172 Geological Society : — 



Practical Geology. The line grey powder is gritty to the touch, 

 and it all passed through a sieve with 30 meshes to the inch. It 

 contains plagioclase-felspar (generally idiomorphic labradorite) coated 

 with a thin film of glass, hypersthene and monoclinic brownish 

 augite, both frequently in perfect crystals, magnetite, apatite, 

 possibly zircon, and fragments of a brown glass. Among: the finest 

 debris there is much felspar in the form of minute chips. The 

 perfect crystalline form of many of the constituents of the dust and 

 the small amount of glass adherent to them, indicate that at the 

 time of projection the glassy magma must have been very fluid, and 

 it must have been to a large extent wiped off the crystals by friction. 

 From Dr. Morris's account, the minerals of high specific gravity 

 appear to have fallen first ; the order being magnetite and pyroxenes 

 first, next the felspars, and finally the glass threads and minute 

 felspar-debris. Dr. Pollard's analysis is as follows: — Si0 o = 52*81, 

 Ti0 2 =-95, Al 2 3 = 18-79, Fe 3 = 3-28, FeO = 4'58, MnO = -28,. 

 (CoXi)O = -07, CaO=9-58, MgO = 5-19, K o = -60, K"a„0 = 3-23, 

 P 2 5 =-15, S0 3 =-33, Cl=-14, H 2 0=-37: Total 100-35. " 



June 11th.— Prof. Charles Lapworth, LL.D., F.E.S., 

 President, in the Chair. 

 The following communications were read : — 



1. ' A Descriptive Outline of the Plutonic Complex of Central 

 Anglesey.' By Charles Callaway, D.Sc, M.A., F.G,S. 



The central complex of Anglesey was originally composed of 

 diorite, felsite, and granite. The gneiss and granitoid rock of the 

 area, formerly regarded as sedimentary in origin, are now known to 

 be plutonic masses. The diorite undergoes numerous modifications, 

 into hornblende-gneiss, chlorite-gneiss, micaceo-chloritic gneiss, 

 and kersantite and biotite-gneiss. The felsite has not been found 

 in its original state, but is converted into ' halleflinta,' quartz- 

 schist, mica-schist, and mica-gneiss ; granite and quartz-felsite 

 are intrusive into the diorite and felsite, and the two former are 

 regarded as derived from the same magma. They are not foli- 

 ated, and "were intruded subsequently to the modification of the 

 diorite and felsite into gneisses and schists. The diorite, ori- 

 ginally a xenolith surrounded and injected by granite, has been 

 modified into an elliptical dome of dark gneiss : into simple gneisses 

 by pressure, and into complex gneisses by pressure plus granitic 

 intrusion. This intrusion has often produced fusion at the contact, 

 sometimes with the generation of biotite in the diorite. In addi- 

 tion to this, the diorite possesses an imperfect fluxion-structure. 



2. ' Alpine Valleys in Relation to Glaciers.' By Prof. T. G„ 

 Bonney, D.Sc, LL.D., P.R.S., F.G.S. 



The author discusses some hypotheses about the formation of 

 Alpine valleys which have been advanced by Prof. W. M. Davis, 

 but has left the Ticino Valley, on which the latter Jays much stress, 

 to Prof. Garwood, who has very lately visited it. Prof. Davis 

 maintains that the upper and wider parts of Alpine valleys were 

 excavated in pre-Glacial times, the lower and narrower portions 



