564 



H. W. FEILDEN AND C. E. DE RANCE ON 



uplands. The destruction of rock is not merely confined to these par- 

 ticular lines of erosion, but is continuous over the whole surface of 

 the country during the episodes of summer thaw. In those climates 

 the approach of winter is not marked by any transition. Beyond 

 the limits of perpetual snow the land at the close of the transient 

 summer is saturated with moisture ; the fissured precipice, the cleaved 

 slate, and the gravel-bed are alike charged with water to their utmost 

 capacity; without any warning, winter lays her icy hand on the 

 scene, and in a few hours the face of nature is changed, and moisture 

 and running water is converted into ice. 



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The destructive force exercised upon rocks during the progress of 

 expansion throughout the entire Polar region is a most potent 

 factor, and gives results but little comparable with the subaerial work 



