1870 GROWTH OF POLAR ICE. 57 



vvhich may take place on its surface by the change of 

 the snow into ice or otherwise. 



' If the ice increases superficially it is difficult to 

 account for the absence of annual lines of stratifi- 

 cation, or a thick stratum of pure fresh-water ice on 

 the upper surface of the floes. In no case have we 

 found the layer of fresh- water ice to be more than 

 about two feet in thickness. It is only to be found 

 in the hollows on the surface of a floe ; the ice at 

 the highest parts, above where the water produced 

 by the melting of the snow would naturally collect, is 

 invariably more or less brackish. 



' Wherever a piece of a floe has been turned on its 

 side, and when in that position become re-frozen into 

 the pack, that part of its former upper surface which 

 was composed of fresh-water ice changes its character 

 and becomes brackish ice and appears as a vertical 

 vein running through the newly formed floe ; such 

 veins never present the decided blue tint which is so 

 frequently to be seen in an iceberg where a crack in 

 the parent glacier has become filled with frozen water. 



' There is, however, evidence that the layer of snow 

 on the surface of the ice does become changed into ice 

 under certain circumstances. On one of the large 

 fioebergs in the ]3ack near the ship a quantity of debris 

 ice had become piled up to a height of eighteen feet 

 above the snow layer of a previous season, which was 

 about two feet in thickness. In March the snow 

 immediately under the piled up hummocks had become 

 changed into ice while that left uncovered remained 

 unchanged. Although no measurement was obtained 

 the thickness of the layer which had changed its for- 



