Land Ice of Arctic and Sub-Arctic Regions 43 



the same slope. In another case a crystosphene was encountered 

 on a mining- claim three feet below the surface, and it was traced 

 for five or six hundred feet down the valley, being everywhere at 

 practically the same depth, while the surface itself had a slope of 

 about one in a hundred, so that this apparently level sheet of clear 

 ice was five or six feet higher at its upper end than at its lower. 

 Examples of this kind could be multiplied almost indefinitely, show- 

 ing plainly that these ice sheets do not partake of the character and 

 attitude of frozen ponds or lakes. 



" While these crystosphenes, or so-called ' glaciers,' are usually of 

 the nature of nearly horizontal sheets, occasionally they occur as 

 veins or dikes of ice rising through the bed rock into the overlying 



gravel More or less vertical masses of ice are also sometimes 



met with in the gravels themselves, indicating the positions of former 

 water channels from the bed rock toward the surface." 



" In the majority of cases crystosphenes are in the vicinity of 

 springs that can be plainly seen issuing from the bases of the neigh- 

 boring hills, but in other cases no such springs are apparent. In 

 these latter cases, however, wherever the gravel has been removed, 

 and the underlying rock has been exposed, springs have been found." 



" The mode of formation of these underground sheets of ice is 

 therefore somewhat as follows : 



" Water, issuing from the rock beneath a layer of alluvial material, 

 rises through the alluvium, and in summer spreads out on the sur- 

 face, tending to keep it constantly wet over a considerable area. In 

 winter if the flow of water is large, and the surface consists of inco- 

 herent gravel, the water will still rise to the surface, and there form 

 a mound of ice. If, on the contrary, the flow from the spring is not 

 large, and the ground is covered with a coherent mass of vegetable 

 material, such as is formed by a sphagnum bog, the spring water, 

 already at a temperature of 32° F., rises until it comes within the 

 influence of the low temperature of the atmosphere above, and 

 freezes. This process goes on, the ice continuing to form downward 

 as the cold of the winter increases, until, a few feet below the sur- 

 face, but still within the influence of the low external temperature, 

 a plane of weakness is reached in the stratified and frozen vegetable 

 or alluvial deposit, such planes of weakness being generally deter- 

 mined by the presence of thin bands of silt or fine sand. 



" As any outlet to the top is now permanently blocked, the water 

 is forced along this plane of weakness, and there freezes ; and thus 

 the horizontal extension of the sheet of ice is begun. While thus 

 increasins: in extent, the ice also increases in thickness bv additions 



