Geology 



gravel Now, if the stone or layer of sand be too thick 

 to get warmed through in the course of a day, it will 

 have a protecting influence on the ice beneath, but in 

 the case of small stones or thin layers of sand the effect is 

 otherwise, as the rocky materials absorb the solar rays and 

 transmit the heat to the ice below, which is thus melted. 



Individual pebbles may often be seen at the bottom 

 of round holes little bigger than the diameter of the 

 pebble. A thin layer of sand also seems to have a 

 similar effect, a thin deposit of this material being found 

 at the bottom of all the shallow pools which abound on 

 the surface. 



Large stones, too thick to get hot through in the 

 day, act as a sunshade and protect the ice beneath them 

 from the heat rays, and thus after a time, as the sur- 

 rounding unprotected ice slowly melts, they are left 

 standing on a short pillar of ice, forming what are known 

 as glacier-tables (Plate XIII.). 



The thick layers of sand and stones forming the 

 rock-trains have a similar effect, and thus the ice beneath 

 them remains as a ridge, as the general surface is 

 lowered. These ridges become higher and steeper as 

 time goes on, until at length the ddbris slides down their 

 sides, forming two parallel ridges. The central ridge, 

 now deprived of its protective covering, soon wastes 

 away, while the lateral ridges afterwards go through 

 a similar evolution, and thus one original rock-train may 

 become split into a large number, and, in fact, in some 

 instances the debris becomes finally almost evenly dis- 

 tributed over the whole surface, near the termination. 



The rock-trains evidently originated in the upper 

 part of the glacier, and we had, therefore, to postpone 



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