Land Ice of Arctic and Sub-Arctic Regions 41 



ders it evident that if their extinction and the consequent burial of 

 ice beneath the surface takes place in the manner supposed, sheets 

 of ice, probably more or less lenticular in shape, should form a 

 characteristic feature of tundra deposits. The origin of the lake- 

 lets may perhaps be due to the accumulation of snow banks on the 

 tundra which by their late melting enable the moss surrounding them 

 to grow more rapidly than on the more deeply covered areas. In 

 this way a depression in the surface would be formed which would 

 be flooded after the snow melted. A lakelet once started would 

 perpetuate itself from year to year until the growth of moss from 

 the sides led to its burial. An origin of this nature seems probable, 

 as the lake basins (in the tundra of Hood- plains) are due entirely 

 to variations in the surface growth of vegetation and not to irregu- 

 larities of the substratum of rock or clay on which the humus layer 

 of the tundra rests. The origin and extinction of lakelets is thus 

 a part of the normal growth of the frozen moss-covered plains." 



Thus it appears the lower level and higher level forms of ice differ 

 in that the presence of the newer is due largely " to variations in 

 the surface growth of the vegetation," while the presence of the 

 older elevated form is due to inequalities caused by slight deforma- 

 tions of the impervious clay deposits of the Pleistocene lake basins. 



One phase of the occurrence of ice sheets in flood-plain alluviums 

 which is of wide distribution and much interest is described by J. B. 

 Tyrrell " in an article entitled " Crystosphenes or Buried Sheets of 

 Ice in the Tundra of Northern America." As he describes the mode 

 of occurrence of the ice under circumstances where it is most often 

 encountered and observed — the placer mining districts of the 

 region — the following is quoted : 



" The Klondike gold-bearing district, to which my observations 

 have lately been confined, and in which the deductions here set down 

 were drawn, is a part of a great unglaciated belt or tract of country 

 lying near the middle of the Yukon Territory in Canada, between 

 the glaciated region which extends on both sides of the ' Chilcat ' 

 or Coast Range of mountains to the south and southwest, and the 

 also glaciated region of the Ogilvie or Rocky Mountain range to 

 the north and northeast. It is a country of high, well-rounded hills 

 and deep, though flaring, valleys, in the bottoms of which flow 

 streams with regularly decreasing grades. On one or both sides of 

 these streams are everywhere deposits of alluvial material, varying 

 from ten to a hundred feet in depth, consisting below of coarse sand 



'^Jour. of Geol., Vol. XII, p. 22,2, 1904. 



