236 J- B. TYRRELL 



stream was found issuing from the rock several feet below the level 

 of the top of the alluvial deposits. This was the source of the water 

 that had formed the ice. 



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 till 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 surface, but still 

 within the influence of the low external temperature, a plane of weak- 

 ness is reached in the stratified and frozen vegetable or alluvial 

 deposit, such planes of weakness being generally determined 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 

 increasing in extent, the ice also increases in thickness by additions 

 from beneath, until it has attained a sufficient thickness so that its 

 bottom plane is beyond the reach of the low atmospheric temperature 

 above ; after which it continues to increase in extent, but not in thick- 

 ness or depth. 



With the advent of the warm weather of summer the growth of 

 the crystosphene ceases, but the cold spring water which contin-ues 

 to rise up beneath it has very little power to melt it, and its covering 

 of moss or muck, being an excellent non-conductor of heat, protects 

 it from the sun and wind, and preVents it from thawing and dis- 

 appearing. Thus at the advent of another winter it is ready for 



still greater growth. 



J. B. Tyrrell. 



Dawson, Yukon Territory, Canada, 

 February 13, 1904. 



