Geological Society of London. 183 



these abnormal markings as due to floating-ice, during the great period 

 of interglacial submergence. 



The moraines were all believed to belong to the last set of glaciers. 



The subject of the " Glacial Origin of Lake-basins" was then entered 

 upon, and the following lakes discussed by means of diagrams drawn 

 to scale, showing lake-depths, mountain -outlines, and the thickness of 

 the ice : — Wastwater, Grrasmere, Easdale, Windermere, Coniston, and 

 Esthwaite, together with several mountain tarns. In the case of Wast- 

 water, the bottom was shown to run below the level of the sea for a 

 distance of a mile and a quarter, and the deepest point to be just oppo- 

 site the spot at which the only side valley joins the main one. While 

 the greatest depth of the lake is 251 ft., the thickness of the old 

 glacier-ice must have been fully 1500, and, all points considered, Prof. 

 Ramsay's theory of glacial erosion seemed to the author certainly to be 

 upheld. In like manner the same theory was thought to account for 

 the origin of the other lakes mentioned, such ones as Windermere and 

 Coniston being but long narrow grooves formed at the bottom of pre- 

 existing valleys. 



Mountain tarns were held to be due sometimes wholly to glacial 

 erosion, sometimes to this combined with a moraine dam, and occa- 

 sionally to the ponding back of water by moraines alone, or moraine- 

 like mounds formed at the foot of snow-slopes. 



II. — February 10, 1875. — John Evans, Esq., E.E.S., President, in 

 the Chair. The following communications were read: — 



1. "The Phosphorite Deposits of North Wales." By D. C. Davies, 

 Esq., F.G.S. 



The deposit of phosphate of lime described by the author is a bed 

 varying from 10 to 15 inches in thickness, which occurs at the top of 

 the Bala limestone over a considerable district in North Wales, having 

 been detected in various localities from Llanfyllin to the hills north and 

 west of Dinas Mawddy. The bed is rendered black by the presence of 

 graphite, and appears to consist of concretions of various sizes cemented 

 together by a black matrix. The concretions are richest in phosphate 

 of lime, some of them containing 64 per cent. ; the average amount in 

 the bed, including the matrix, is 46 per cent. The deposit is under- 

 lain by a bed of crystalline limestone, and sometimes divided by thin 

 beds of similar limestone into two or three layers. The author noticed 

 the principal fossils occurring in the Bala limestone below the phos- 

 phorite beds, and stated that many of those in the overlying shales, up 

 to a certain distance above the bed, are phosphatized. The author 

 referred to the presence of phosphate of lime in the inner layers of 

 Unio and Anodonta to the amount of as much as 15 per cent., and 

 thought that the phosphate of lime in the deposit was probably of 

 organic origin. It may have been an old sea-bottom on which the 

 phosphate of lime of Mollusca and Crustacea was accumulated during 

 a long period, and seaweeds may also have contributed their share. It 

 probably represented the remains of an ancient Laminarian zone. The 

 author suggested that the phosphatic nodules of the so-called coprolite 

 beds in other parts of England might have been derived from the 

 denudation of similar deposits. 



