272 WALKS AND TALKS. 



year, the line of permanent snow extended itself southward. 

 Probably the volume of snowy precipitation was increased, 

 and thus the march of the reign of snow, was accelerated. 



The sheet of perpetual snow, to whatever limit it reached, 

 was divided into two areas by an isothermal line. Had the sun 

 never exerted a thawing influence had no thawing ever 

 taken place in any portion of the snow, it would have re- 

 mained a soft and fleecy covering. But wherever incipient 

 thawing was felt, the snow crystals began to resolve them- 

 selves into grains of ice. Down to the latitude where this 

 change was unable to proceed farther, the condition of the 

 snow remained granular, as it now does in the Alps. If how- 

 ever, the melting influence proceeded farther, the granular 

 snow-mass resolved itself into solid ice. These changes can be 

 traced in the snow which falls upon our streets. Thus all the 

 southern portion of the snow-field became a true glacier; 

 north of that was a zone of neve ; and possibly a zone of soft 

 snow covered the area still nearer the pole. All this, of 

 course, supposes precipitation to have taken place. If in any 

 region, precipitation was wanting or scant, the snowy or icy 

 covering did not appear. 



On its northern border, the glacier was fixed to the mass 

 of ice or neve beyond ; and very probably it was fixed to the 

 earth. The glacier, like all glaciers, must move; and the 

 motion would be developed along the free border. The 

 glacier, therefore, traveled southward. Consider the conse- 

 quences of the motion. The soft snow had filled the gorges 

 and the river valleys. It had settled around cliff and crag, 

 and when it became ice, it held them in its firm grasp. The 

 motion of the glacier wrenched fragments from their fasten- 

 ings and moved them southward. The rock fragments, like 

 diamonds in a setting, marked and scored the underlying sur- 

 face. The loose materials, the accumulation of a previous 

 geologic aBon, were plowed to the bed-rock. The bed-rock 

 was scored and striated by the tremendous power of the 

 glacier. 



Every year, the great ice-sheet encroached a little farther 



