Geological Society of London. 333 



Part III. Origin of the Stri^. 



The stria? are not such as can have been produced by valley- 

 glaciers ; they go across and not down the valleys, nor are there 

 any moraines. The question resolves itself into (1) the hypothesis 

 of two ice-sheets moving in different directions in the two areas ; 

 (2) that of floating-ice. The first is opposed by the facts that the 

 rock-surface is not moutonneed on a lai'ge scale, and that the stria? and 

 terminal curvature are far from universal ; that the drifts associated 

 with the stria? are marine deposits ; that stria? having different 

 directions are found on the same slab. The well-known occurrence 

 of gravel-ore in the drift at the outcrop of a vein is also against this 

 hypothesis. The marine origin of the drifts is indicated by their 

 well-marked stratification as a whole, by the alternations of well- 

 washed sands and gravels with the Boulder-clays, and by the occur- 

 rence through all the beds of marine shells. A lower or basement- 

 clay is seen in places under this marine drift, but it is always the 

 latter with which the stria? are associated. The great development 

 of undoubted marine beds and comparative rarity of moutonneed sur- 

 faces constitute the principal differences between this region and 

 the north, where the existence of an ice-sheet has been strongly 

 advocated. Anglesey is considered by Sir A. Kamsay to have 

 received its configuration by the action of an ice-sheet from the 

 north ; but its physical features appear to be due rather to its geo- 

 logical structure, and to have existed in more or less their present 

 form in pre -glacial times. 



The arrangement of drifts in this district presents an analogy with 

 that of the Norfolk drifts, and probably results from a similar 

 sequence of events. 



The marine drifts, from their great variability, seem to have been 

 distributed, and the striations produced by floating-ice, driven by 

 tidal or oceanic currents, during the time of submergence. During 

 this time Snowdon and the surrounding hills must have stood well 

 above water, forming an island-group, and by such a group the pre- 

 vailing currents from the north would be deflected to the south-west 

 over Anglesey on the one side, and to the south-east over the plains 

 of Cheshire and Shropshire on the other, while within the limits of 

 the group a local circulation might be maintained. 



II.— June 9, 1886.— Prof. J. W. Judd, F.R.S., President, in the 

 Chair. — The following communications were read : — 



1. " On the Volcanic Rocks of North-eastern Fife." By James 

 Durham, Esq., F.G.S., with an Appendix by the President. 



After describing the general distribution of the volcanic rocks of 

 Old Red Sandstone and Carboniferous age in the counties of Forfar 

 and Fife, the author called attention to a fine section exhibited 

 where the Ochil Hills terminate along the southern shore of the 

 Firth of Tay. In immediate proximity to the Tay Bridge, a series 

 of the later volcanic rocks, consisting of felstones, breccias, and 

 ashy sandstones, are found let down by faults in the midst of the 



