644 



E. DE K. LEFFINGWELL 



The upper surface of the ice is usually less than two feet under 

 the ground in muck formations, about the limit to which the 

 summer's thawing penetrates. This surface is usually horizontal, 

 or undulating with the surface of the ground. One or two exposures 

 showed a dome-shaped surface, and another, a central projection 

 above the general surface (Fig. 9). Some more complicated expo- 

 sures are shown in Figs. 12 and 13. The overlying material is 

 usually muck capped by a few inches of turf. Occasionally it is 

 peat capped by growing sphagnum ( ?) moss. 



Without going deeply into the question of the crystallization of 

 ice, it may be remarked that ice resulting from snow (and glaciers) 



Fig. II. — A large ice wedge which spreads out under the surface of the ground. 

 The vertical lines indicate rows of whiter ice full of air bubbles. The material on 

 either side is sand. To the right are upturned muck beds. 



is granulated, but that from standing fresh water is vertically 

 prismatic. Sea-ice is different from both, but very little has been 

 written upon the subject. The writer's own observations are that 

 sea-ice loses its salt at temperatures approaching 0° C. and becomes 

 honeycombed, showing a general vertical structure, but decidedly 

 different from that of fresh-water ice. 



A fresh transverse section of an ice wedge shows a face of whit- 

 ish ice with numerous parallel vertical markings (Figs. 11-14). 

 These markings are usually of whiter ice which is seen to contain 

 an unusual amount of air bubbles. It is often visibly granular, yet 

 shows a general vertical structure, and breaks up into short, irregu- 

 lar pieces when allowed to melt slowly in the shade. 



