42 Reports and Proceedings — Geological Society of London. 



new literature on the subject which has appeared since 1903, the date 

 of the second edition, and special notice is devoted to the vertical 

 geological range of particular forms. 



E,E]I=OI?.TS .A-3SriD IPI^.OCEEIDIlSrCB-S. 



I. — Geological Society of London. 



l.—Novemler 22, 191 1.— Professor W. W. Watts, Sc.D., LL.D., M.Sc, 

 r.R.S., President, in the Chair. 



The following communications were read : — 



1. " Petrological jS'otes on Guernsev, Herm, Sark, and Aldernev." 

 By Professor T. G. Konney, Sc.D., LL.D., F.R.S., F.G.S., and the 

 Eev. Edwin Hill, M.A., F.G.S. 



The authors returned to these islands, after an interval of nineteen 

 years, in order to give further study to the relations of the igneous 

 masses and especially of certain dyke-rocks. As they had themselves 

 suspected in 1891, and as Mr. J. Parkinson had announced in 1907, 

 the old distinction between diorite and syenite could not be maintained ; 

 but there existed, especially in Guernsey and Alderney, a dioritic 

 magma which underwent difierentiation. The results of this are 

 described, the most basic being found at Fort Albert (Alderney) and 

 Bon Repos Bay (Guernsey), and the most acid, which are really 

 tonalites, more especially in the north-west of the latter island. 

 These and a felspathic variety sometimes intrude, sometimes pass 

 into the others, so they also must have been at high temperature. 

 The so-called ' granites ' at the two ends of Sark are hornblendic, the 

 southern one being really a tonalite ; so are those of Alderney, Herm, 

 Jethou, and Guernsey, and it is suggested that these granites may be 

 yet more acid terms in a differentiation series. 



Of the numerous dykes the most acid are either aplitic micro- 

 granites or quartz-felsites, the latter being the rarer and probably 

 later in date. Some of the former at Castle Cornet exhibit an 

 interesting structure. Here diorite cuts gneiss, and has partly melted 

 down a little of it. Microgranite has also cut and melted some of 

 the diorite, the result being a streaked red and green rock. Diabase 

 dykes are common, and mica-traps have been found in all the islands 

 except Herm. The former almost invariably cut the microgranitic 

 dykes, but are cut by the quartz-felsites. 



At Pleinmont, in the south-west of Guernsey, a mass resembling 

 a greenstone proves, as mentioned by the late Pere Noury, to be 

 sedimentary. This has been examined and is considered to be, like 

 the ' argillites ' of Jersey, Brioverian in age. It is cut by dykes of 

 quartz-felsite and by one of diabase (older than these). 



The time-relations of the several rocks are discussed. The gneiss 

 of Guernsey, a pressure-modified granite, is the oldest, and had 

 acquired its structure before the intrusion of the diorites. They 

 were followed by the hornblendic granites, and these by the aplitic 

 microgranites. All were pre-Cambrian, and perhaps even the last 

 was older than the Brioverian. The date of the diabase dykes is 

 more uncertain. They are appai-ently earlier than the quartz-felsite 



