206 Geological Society: — 



and Alderney, a diorifcic magma, which underwent differentiation. 

 The results of this are descrihed, 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 jSToury, 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 apparently earlier than the quartz- 

 felsite dykes which may possibly be of the same age as the acid 

 lavas below the conglomerate of Eastern Jersey, now regarded as 

 basal Cambrian; but in Alderney two diabase-dykes cut the gres 

 f eldspathique. There is, however, reason to think that these 

 dykes are of more than one age. The mica-traps are probably late 

 Palaeozoic. 



2. ' The Evolution of Inoceramus in the Cretaceous Period.' By 

 Henry Woods, M.A., F.G.S., Lecturer in Paloeozoology in the 

 University of Cambridge. 



December 6th, 1911.— Prof. W. W. Watts, Sc.D., LL.D., M.Sc, 

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

 The following communication was read : — 



' The Faulted Inlier of Carboniferous Limestone at LTpper 

 Yobster (Somerset).' By Thomas Franklin Sibly, D.Sc, F.G.S. 



The three small masses of Carboniferous Limestone at Luckington, 

 Upper Yobster, and Tor Bock, Yobster, respectively, which lie in 

 the overfolded Coal Measures of the southern portion of the East 



