Iff 8in5. 



^3f!4iri5. 



B. 



C. 



156 GLACIOLOGY 



the lake ice, and the resultant prisms are sharply outlined by air globules forced 

 out between the crystals. Some explanation of this kind is necessary to account 

 for the domed and uneven surface and great thickness of this variety of ice in the 

 southern half of the Blue Lake. This change, of whatever nature it may be, always 

 takes place in the snow of snow-drifts where there is little lateral pressure, and it 

 is quite conceivable that if a molecular change took place, resulting in the formation 

 of well-marked hexagonal prisms, it should be accomjaanied by the extrusion of the 

 air from the ice, and in this air being forced into position as minute bubbles along the 

 lines of natural weakness, that is, between the boundary faces of the different crystals. 

 The top layer of prismatic ice, 4 inches in thickness, as just described, was followed 



by 3 feet 8 inches of ordinary lake ice with some air 

 1 inches. A. bubbles. Beneath the ice was 13 feet of slightly 

 Ijrackish water. 



The bottom material proved to be a deposit of black, 

 evil-smelling mud containing diatoms, some of excep- 

 tionally large size, pieces of decayed alga? of small species, 

 several living species of Rotifera and Tardigrada (Water- 

 bears), and some obscure bodies which may possibly be 

 spores, but have not been determined. This deposit was 

 about 6 inches thick, and was succeeded by the typical 

 morainic gravel. 



Second Trench in Clear Lake, April lith, 1 5th, 16th. 

 D A. 4 inches of prismatic ice. 



B. Ice with air and gas bubbles in it 1 foot 8 

 inches. 



C. 3 feet 4 inches ice with drawn-out bubbles giving 

 Fig. 50 prismatic structure. 



D. 3 feet 9 inches ice with bubbles. 



The difference of thickness of the ice in the two trenches is easily accounted for 

 by the presence near the first trench of the large knoll of intensely black kenyte, 

 the influence of which must have been very considerable in regulating the amount 

 of thaw which had taken place during the summer just past. 



One other feature of this lake worthy of mention was the occurrence at a depth 

 of 2 or 3 feet of what was apparently a well-defined ablation -rippled surface with 

 the depressions between the ripples still filled with snow, and also at the same level 

 of strings of what we called snow-tabloids, lentlcles of air in the ordinary lake ice 

 partially filled with a loose powdery ice. 



Temperature at the Second Trench at Clear Lake. 



3ft9ins 



Mean Air Temperatures, April loth 

 I. II >, 16th 



II II II 17th 



- 10-4° F. 



- 16-5° F. 

 -21-0° F. 



