THICKNESS OF THE GLACIER. 175 



it, tliereforc, that the sheet of ice which moved over them must 

 have been five or six thousand feet thick in order to proceed in 

 a southerly direction without being obstructed by those hills. 



Now let us see how we can account for the general motion 

 from north to south. I need hardly repeat that over the whole 

 of this surface we find solid rock, evenly polished ; we find every- 

 where these scratches and grooves — sometimes a groove a foot 

 in depth and as wide, or a foot and a half in width, and from 

 that down to mere scratches, such as a diamond would make 

 upon glass, all this being the result of the inequality of the 

 dimensions and hardness of the materials which were imbedded 

 in the lower surface of the ice as it moved on. 



But now let us have this as the equator, [making a sketch as 

 he proceeded,] — this as the curve of the northern hemisphere 

 of the earth, — this is the latitude at which only snow falls, — 

 here the latitude at which snow and rain alternate, — and here 

 the latitude at which, now-a-days, only rain falls. The amount 

 of snow which accumulates at the extreme north will not be 

 transformed into ice to the extent to which it will be trans- 

 formed further south. We find greater glaciers in Greenland 

 than we find further north. In one word, we find that the 

 dimensions of the glacier are proportioned to the amount of 

 fluid moisture which may condense from the atmosphere. Now, 

 in the days when the climate of Switzerland was so cold as to 

 cause the formation of a sheet of ice of a thickness five thousand 

 feet greater than any of the glaciers now existing, when the 

 whole plain of Switzerland was under ice, and when the north- 

 ern portions of Europe were covered uniformly by one sheet of 

 ice, moving southwards, the climate of the adjoining continents 

 cannot have been warmer than the climate then prevailing in 

 Europe. We must have had the same intensity of cold on this 

 continent as existed there. We must, therefore, have had a 

 condition favorable to an accumulation of snow in the northern 

 hemisphere, which may have extended much further than any 

 glacier now in existence, and under those conditions I suppose 

 that the sheet of ice might have extended to the United States 

 and to the Gulf of Mexico. I have traced the polished surfaces 

 down to Alabama ; therefore I am satisfied that at one time that 

 sheet of ice extended as far as that. And to have extended so 

 far, we cannot assume that the accumulation of snow in the 



