70 Geological Society. 



these tuffs and sediments, a fact that implies comparative remoteness 

 from the volcanic centre; and the important masses of intrusive 

 matter represented upon the maps are themselves largely composed 

 of the products of explosive action. The numerous sheets of ophitic 

 dolerite, aphanite, and altered andesite, that lie, seemingly inter- 

 bedded, on the. northern slopes, were probably intruded when the 

 associated rocks were already weighed down by much superincum- 

 bent sediment, A common character of these basic sheets is the 

 development of small colourless crystals of epidote. 



The most striking mass upon the mountain is the main " felstone " 

 (eurite) of the wall, which proves to be miuutely " granophyric," 

 and of very uniform grain throughout. An analysis by Mr. T. H. 

 Holland shows 73 per cent, of silica. This vast intrusive sheet is 

 regarded as perhaps of no later date than the Llandeilo lavas of 

 Craig-y-Llam, and as a forerunner of the volcanic conditions that 

 prevailed in Bala times throughout North Wales. 



The stratigraphical horizons, as shown on published sections, 

 would throw a great part of the tuffs and ashes described into the 

 Tremadoc beds, or even lower, in contradiction to the generally 

 accepted statement that volcanic activity began in the Arenig times. 

 While this point can only be settled by detailed mapping on the 

 basis of the new six-inch survey, the authors incline to the belief 

 that the eruptions in this area broke out in the Cambrian rather 

 than the Ordovician period. 



May 8.— W. T. Blanford, LL.D., F.H.S., President, 

 in the Chair. 



The following communications were read : — 



1. " The Eocks of Alderney and the Casquets." By the Rev. 

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



The author in this paper described Alderney, Burhou, with its 

 surrounding reefs, and the remoter cluster of the Casquets, all in- 

 cluded within an area about 10 miles long. 



Alderney itself consists in most part of crystalline igneous rocks, 

 hornblendic granites of varying constitution which resemble some 

 Guernsey rocks, but seem more nearly connected with those of 

 Herm and Sark. These are pierced by various dykes, and among 

 them by an intrusion containing olivine, which may be placed with 

 the group of picrites. There is also in the island a dyke of mica- 

 trap. 



The eastern part only of Alderney, but the whole of Burhou, the 

 Casquets and their neighbouring reefs, consist of stratified rocks. 

 These contain rare beds of fine mudstone, but are generally false- 

 bedded sandstones, and grits, sometimes with pebbles, often rather 

 coarse and angular, occasionally becoming typical arkoses. At a 

 point on the southern cliffs of Alderney they may be seen to rest on 

 the crystalline igneous mass. A series identical in constitution and 

 aspect occurs at Omonville, on the mainland, a few miles east of Cap 

 La Hague (as had also been noticed a few months earlier by 



