16 



A CONTRIBUTION TO THE STUDY OF ICE-STRUCTURES 



8. GLACIER-ICE (for comparison) 



For comparison with the textures of the lake-ices a reference to that of glacier -ice 

 from the vicinity is interesting. 



A sample of the ice at sea-level, about one hundred feet below the surface of the glacier, 

 one mile south of Cape Barne, was found to be composed of equidimensional grains with 

 interlocking and re-entrant straight-line boundaries. Some of the grains were an inch 

 in diameter. It was loaded with bubbles well shown in the photograph (see Plate II, 

 Fig. 4). The bubbles were very irregular in shape, often spherical or dumb-bell-shaped, 

 and sometimes tube-shaped. The spherical bubbles were commonly a sixteenth of an 

 inch in diameter. Where elongated they were found to be leaning at an angle of 45° with 

 the upper end forward in the direction of motion of the glacier. 



Nearer the surface the ice of the glacier passed into a finer grained state. 



9. CLASSIFICATION OF THE CRYSTALLINE TEXTURES OF 



THE LAKE-ICES 



The crystalline texture of the lake-ices has been shown by the foregoing investiga- 

 tions to depend in the following manner upon the proportion of dissolved salts in the 

 water : 



(a) Fresh water allows of the formation of coarse prisms. 



(b) Somewhat saline water yields medium -sized prisms. 



(c) Highly saline water produces plate structure. 



(d) Cryohydrates solidify as a fine-grained mass, in which intersertal 



structure is often prominent. 



III. SEA-ICE 



Refer to The Heart of the Antarctic, Vol. II, p. 337, and Vol. I, p. 180, of this series. 



The ice of the first few inches from the surface usually originates as a felting 

 together of small floating crystals. As the freezing in that layer proceeds more 

 rapidly than in the deeper zone, a larger proportion of brine is trapped in the inter- 

 stices of the feltwork. Below, the ice is prismatic with saline tracts between the prisms. 

 As in the case of lake ices, a horizontal stratification is noticeable throughout the first 

 foot or two below the surface. 



Snow piled upon the surface becomes consolidated, and draws up a small quantity 

 of brine from the salt ice below. The following specific gravity determinations were 

 made from samples got in a shaft sunk by Priestley in the frozen surface of Back Door 

 Bay in the winter : 



Specific gravity of thaw- 

 water at 4° C. 



First foot 

 Second foot 

 Third foot 

 Fourth foot 

 Fifth foot 



A sample of the surface sea-water of McMurdo Sound, taken about the same time, on 

 an occasion after a blizzard had cleared the floe out, had a specific gravity at 4° C. of 

 10275. 



