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PROE. J. PRESTWICH ON THE GLACIAL PERIOD, WITH 



when there was a submergence of from 1500 to 2000 feet, and a 

 large land-area became Covered by the sea — a change which could 

 hardly have failed to affect the climate not only of those regions, 

 but also of Switzerland and other parts of Europe. It is not my 

 intention, however, to say more at present upon this question, but 

 to proceed to the consideration of the third and main point, namely, 

 the probable duration of the Glacial epoch, which hinges so materially 

 on the laws affecting ice- growth and glacier-motion. 



Twenty years ago the only data we had bearing on the subject of 

 glacier-motion and action were the observations of Agassiz, Eorbes, 

 Tyndall, and others on the glaciers of the Alps. These had taught 

 us the rate of motion of many of the Alpine glaciers, their periods 

 of advance, and the causes of their growth and decay. It was on 

 these facts alone, which are really not applicable, that we had to 

 judge what might have been the possible rate at which the great 

 ice-sheet spread over the land. 



It was found that in July the Mer de Glace advanced at the rate 

 of 33 inches in the twenty-four hours, and the Aletsch glacier 19 

 inches in August ; whilst the winter rate was estimated at about 

 half that of summer ; so that roughly the rate of motion for the 

 year may lie between 300 and 400 feet, though in one year the Mer 

 de Glace was found to have advanced 483 feet. 



The position of Hugi's hut on the Aar glacier gave a more definite 

 measure based on a longer average, for in the course of fourteen 

 years it was found to have been carried at the mean rate of 338 feet 

 annually. We may therefore take the average motion of the 

 Swiss glaciers to be about equal to 300 or 400 feet annually. 



The advance and retreat of the terminal front of a glacier are 

 of course independent of this continuous motion of the great body 

 of ice, and depend on the climatal conditions of the year. In 

 the cold summers of 1816 and 1817 there was a general advance 

 of all the Swiss glaciers, whilst since 1856 there has been a general 

 retreat. Usually the loss or gain is small, but in some years 

 it is considerable. In the two years above named some of the 

 glaciers advanced from 100 to 150 feet or more. Of other Swiss 

 glaciers an advance of 1 metre daily in summer is recorded ; but the 

 most remarkable case is that of the Yernagt glacier in the Tyro- 

 lese Alps, which is exceptional in that it advances by fits and 

 starts. Between 1843 and 1847 the ice from this glacier covered 

 the valley below to the length of 1264 metres, which gives a mean 

 annual rate of 870 feet, while the thickness of the mass of ice near 

 »ts extremity exceeded 500 feet. Another glacier in the same 

 range advanced after all oscillations above a mile in a century. On 

 the other hand the Rhone glacier between 1856 and 1877 retreated 

 nearly half a mile, or on an average 116 feet annually; whilst 

 between 1870 and 1877 the retreat extended for a distance of 400' 

 metres, or a mean of 187 feet per annum, the greatest retreat being 

 between the years 1870 and 1874, when it amounted to 250 metres, 

 or a mean of 205 feet per annum. In the Yalley of Chamouni the 

 Glacier du Tour retreated 320 metres in the eleven years between 



