Correspondence — Major Brickenden. 479 



an " adequate vis a tergo," and says : " That such a vis a tergo can 

 only, in the case of such ice-sheets, be obtained by postulating a 

 considerable slope to the ice surface is equally plain and equally 

 admitted." Will Sir Henry H. Howorth kindly express the necessary 

 slope to produce motion in degrees, and give the yield-point of ice 

 by which this slope is controlled ? 



I would have answered Sir Henry H. Howorth before, but have 

 been for a short tour in Switzerland with Mr. G. Fletcher, F.G.S. 

 Shortly we hope to be able to put together some notes we then made 

 on the structure of glacier ice. During our stay I was able to 

 examine many of the larger glaciers, and also to reach the summit 

 of Mont Blanc and compare true neve with true glacier ice. 



E. M. Deelet. 



CHANGES IN OLD LAND-SUEFACES AND COAST-LINES IN THE 

 GLACIAL PERIOD. 



Sir, — For some time past I have been attempting to represent, 

 by coloured diagrams, the gradual changes of the level of the land 

 above the sea, and the relative altitude of the Hills in the British 

 Islands in Pleistocene times, and up to the present period. It was, 

 in my opinion, a great and prolonged denudation which took place 

 during the first Glacial epoch, and after the maximum of glaciation 

 was reached, a period of land submergence was also effected, and 

 then commenced a gradual emergence, in which a wearing away in 

 depressed areas progressed by action of the sea, leaving high-level 

 drifts on the surface of Chalk and other rock formations ; but the 

 channels which now divide the French and English coasts, and the 

 Isle of Wight from the mainland of Hampshire, had not been cut, 

 and did not prevent the migration of the terrestrial inhabitants, the 

 temperature reverting to a condition similar to that now prevailing. 

 The inter-Glacial period, which we now designate as Palajolithic and 

 Prehistoric, ensued. It was a mild climate which then prevailed, 

 and continued for about 15,000 years duration, when there existed 

 a fauna composed of Elephants, Ehinoceroses, Hippopotami, Bison, 

 Musk-Ox, Irish Deer, Elks, Red Deer, Beavers, Wild Boars, Bears, 

 Cave Lions, Hyeenas, and other Carnivora. These creatures ranged 

 through vast forests of Oaks, Elms, Yews, Fir Trees, with peat- 

 mosses separating Arctic plants beneath from a flora of warmer 

 times above. 



At this time a race of men appeared whose intelligence was 

 sufficient to enable them to provide for their wants and to engrave 

 figures of animals and make v^^eapons of stone and bone. We find 

 their relics in caves and under the shelter of rocks, which have 

 resisted destruction during the subsequent submergence to which 

 they were exposed, and which must have continued for a period of 

 about 15,000 years. 



An ice-sheet covered the land, whilst it was submerged, to the 

 depth of many hundred feet, and of this we have records in Scandi- 

 navia, in Canada, and also on Snowdon. 



