CAUSE OF GLACIER MOTION. 



glacial origin of lake-basins point out that the length 

 of those basins in proportion to their depth is so great 

 that the slope up which the ice has to pass is in reality 

 but small. This, no doubt, is true of lake-basins in 

 general, but it does not hold universally true. But the 

 theory here advocated does not demand that an ice- 

 formed lake-basin cannot have steep sides. We have 

 incontestible evidence that ice will pass up a steep 

 slope ; and, if ice can pass up a steep slope, it can exca- 

 vate a basin with a steep slope. That ice will ascend 

 a steep slope is proved by the fact that comparatively 

 deep and narrow river- valleys, such as that of the Tay 

 in some places,* are found often striated across. Hills, 

 also, which stood directly in the path of the ice of the 

 Glacial Epoch are sometimes found striated upivarcls 

 from their base to their summit. 



From what has been already stated in reference to 

 the re-solidifying of the particles in the interstices of 

 the ice, the application of the theory to the explanation 

 of the effects under consideration will no doubt be 

 apparent. Take the case of the passage of the ice- 

 sheet across a river-valley. As the upper surface of 

 the ice-sheet is constantly receiving heat from the sun 

 and the air in contact with it, there is consequently a 

 transference of heat from above downwards to the 

 bottom of the sheet. This transference of heat is 

 accompanied by the melting and re-solidifying of suc- 

 cessive particles in the manner already detailed. As 

 the fluid particles tend to flow into adjoining inter- 

 stices before solidifying and assuming the crystalline 

 form, the interstices of the ice at the bottom of the 

 valley are constantly being filled by fluid particles 

 from above. These particles no sooner enter the inter- 

 stices than they pass into the crystalline form, and 



* See < Climate and Time,' p. o'2G. 



