182 THE POPULAR SCIENCE MONTHLY. 



of the briefest sunshine. Besides this, the earth is 10,500,000 miles 

 farther from the sun in winter than in summer. According to the 

 most careful calculations, the temperature of extreme northern re- 

 gions would be lowered 50, and the mean annual range would be 

 fully 60 below zero. This in all probability would carry the isother- 

 mal line of Labrador, South Greenland, and Iceland (32 Fahr.),down 

 to Charleston and the Gulf of Mexico. The late Prof. Agassiz found 

 ice-marks as far south as this, though it can hardly be supposed that 

 the permanent glacier extended so far. There are, however, abun- 

 dant signs of the permanent ice-layer all over the State of New York, 

 and both east and west of it. The same distinguished authority was 

 wont to claim in his lectures that all the beautiful north and south 

 lakes of Western New York the Cayuga, the Seneca, the Canandai- 

 gua were ploughed out of the solid rock and walled around with their 

 clay and gravel hills by advancing and retreating glaciers. The rocky 

 summits of New England are found to be grooved and scored all over 

 their sides and tops with markings always in nearly a north and south 

 direction. They have been traced on Mount Washington to within 

 300 feet of the highest point. There can be no doubt that at the time 

 we are writing of, about 200,000 years ago, there was one solid ice- 

 stratum of immense thickness Agassiz said from two to three miles 

 slowly being pushed from the northward by the power of freez- 

 ing water, over all of New England and the lake States. 



Again the perigee proceeds to meet the autumnal equinox. The 

 winter and the summer seasons have again become equal in length; 

 and the sun is just half its time on the north side of the .equator. The 

 great ice-shroud is now being gradually withdrawn. Where it abuts 

 on deep waters, enormous icebergs are broken off and float away to 

 the south, carrying bowlders and soil and whatever it may have picked 

 up in its slow course down to the sea. Where it terminates in shallow 

 waters or on the land, its effect is to produce such an arrangement 

 and diversity of soils and such a peculiar outline of country as no 

 other agency could ever have brought about. So different is the na- 

 ture and work of the great polar glacier from anything with which we 

 are familiar at the present day, that it has seemed to me to require a 

 few words of more particular description. 



As is well known, the glacier is an accumulation of many winters' 

 snows consolidated by pressure into a clear blue ice. In this condi- 

 tion it manifests the peculiar property of viscous bodies it is in con- 

 tinual slow motion in the direction of least resistance. Whether it is 

 by the expansion produced by the repeated thawing and freezing of 

 water in its interstices, as Agassiz claimed, or whether by the press- 

 ure of the mass and glacial regelation, which is the constant freezing 

 together of ice-surfaces in contact, after breaking under unequal press- 

 ures, or crashing against obstacles, which is the theory of Prof. 

 Tyndall, or whether by both causes combined, certain it is that large 



