44 Smithsonian Exploration in Alaska in 1904 



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

 bottom plane is beyond the reach of the low atmospheric tempera- 

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

 in thickness or depth. 



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

 of the crystosphene ceases, but the cold spring water which con- 

 tinues 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 

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

 still greater growth." 



Ice sheets in alluvium bottoms on gradients appear to bear a cer- 

 tain relation to the isothermal surface marking the plane of division 

 between the constantly frozen substratum and the annually thawed 

 superstratum or surface layer. This thawed surface layer, com- 

 posed in most cases of peat and growing sphagnum, or tundra 

 (" Cryptogamic plants make more than nine-tenths of its mass, and 

 on their power to grow above as they die and decay below depends 

 the existence of the tundra.") lies like a tenacious wet blanket over 

 the stable frozen substratum of alluvium or bed rock as the case 

 may be. This blanket-like surface layer even where developed only 

 to the extent of an acre or so is conducive to a condition of capillary 

 saturation and hydrostatic semi-flotation. For in these Arctic 

 regions the cold of winter penetrates into the saturated earth and 

 converts it annually into a solidly frozen mass. However warm 

 the short summers may be it is insufficient to melt more than a super- 

 ficial portion of this boreal blanket, so only a swampy carpet of moss 

 may flourish upon the constantly frozen substratum. Through this 

 the standing water cannot sink. As the weather is never warm 

 enough to carry it off by evaporation, these marshes extend far and 

 wide, even up the sides of the hills and mountains. 



An hypothesis to explain the occurrence of ice sheets under a 

 mantle of moss under some of the circumstances where it is met 

 with, especially on sloping surfaces such as Tyrrell describes for 

 the Klondike region and which are common elsewhere in Alaska, is 

 similar to a suggestion made by Lieutenant Belcher.*^ 



The water sinks through the moss blanket from the surface and 

 also seeps underneath it from higher levels. This tends to lift the 

 living moss with its thawed underlying layer of vegetable humus or 



"Beechey's Voyage to the Pacific and Beering's Strait, Vol. I, pt. II. 

 Appendix, p. 600. 



