A CONTRIBUTION TO THE STUDY OF ICE-STRUCTURES 15 



(1 ) To a depth of eighteen inches the ice had a strongly marked stratified appear- 

 ance, owing to the presence of horizontal sheets of gas-bubbles at intervals of half an 

 inch to one inch (see Plate III). In the lower portions the layers were closer 

 together and less well defined. The individual crystals were prismatic and of larger 

 dimensions in the deeper layers. They measured half an inch to one inch across 

 and from one inch to two inches, occasionally reaching six inches in length. They were 

 bounded by straight- fine faces but showed frequently interlocking junctions. In the 

 deeper layers the bubbles tended to become elongated in the vertical, usually resulting 

 in a tube with a bulbous extremity. The needle-like tubes had a diameter not exceed- 

 ing one thirty -second of an inch. 



(2) Next came a six -inch layer of slightly odoriferous saline ice, with salt tracts 

 between the prisms. 



(3) Then there was a nine-inch layer which, in the hand-specimen, had the ap- 

 pearance of jet, and was found to be composed of the peat-like remains of the alga 

 already referred to, with a small admixture of grit, all cemented by twenty -five per cent, 

 in bulk of frozen, somewhat briny water. For many years previously this level must 

 have represented the downward limit of thaw. 



(4) Below the main band of algal peat was a belt one foot four inches thick, consti- 

 tuted of layers usually from half an inch to two inches thick, charged with more algal 

 remains alternating with other clear ice-bands. The last four inches of this section were 

 composed of an extra thick band of algal peat and rested on the bottom of the lake. 

 In the layers where organic remains were abundant the ice tended towards an even- 

 sized, granular texture, in which the grains were polygonal, averaging a quarter to a 

 third of an inch in diameter. Where there was little peaty matter the prism structure 

 was preserved and extended from one layer to the other. 



7. CLEAR LAKE 



Refer to Vol. I, p. 154. An elongated lake about one hundred yards in length and 

 thirteen feet deep where trenched. It was particularly noted for the large area of its 

 surface, formed of handsomely marked coralloidal ice. In other places snow-drifts lay 

 on the surface, and beneath them could be traced stages in the formation of the coral- 

 loidal ice. Where the second shaft was sunk, in April, four inches of the figured ice 

 at the surface passed down into prismatic ice in which were vertical columns of 

 bubbles, then bubbles strung like beads, and finally isolated bubbles in vertical 

 fines. Prismatic structure was marked from the coralloidal surface down, but beyond 

 the two-feet level became more prominent, and vertical gas-tubes, some of them 

 negative crystals, appeared. This latter was no doubt ordinary lake ice, but that 

 above was formed in some other way, partially or wholly, from snow-drift accumulations. 

 At five feet four inches there appeared to be another line of demarcation, for the ice below 

 showed only occasional bubbles, was very transparent, and exhibited a conchoidal 

 fracture. At six feet the crystals were still prismatic, bounded by polygonal faces, 

 averaging a quarter of an inch in diameter, but from five inches to six inches long. 

 Often several adjacent crystals were optically continuous, producing faces half an inch 

 or more across. 



The total thickness of the ice was nine feet one inch, and there were some four feet 

 of water below it. The specific gravity of that water at 4° C. was 10014. 



