Reports & Proceedings — Geological Society of London. 555 



movement, very well illustrated by the fact of its ploughing through 

 the other shelf-ice with such force that a shatter-zone some miles 

 wide is developed. 



The wall of the shelf-ice on the west side offers an excellent 

 example for study, as it is a section from the point of its departure 

 from the land to its crumbling apex. In the case of the Koss 

 Barrier, the cliff-face is a section across the direction of movement. 



At the land end, the Sliackleton Shelf, from the surface down, is 

 hard glacier-ice breaking with a characteristic fracture. A few 

 miles farther out, away from the influence of the winds descending 

 from the land slopes, a neve mantle commences to make its appear- 

 ance over the original ice-formation. As one steams along the face 

 away from the land, this capping is observed to increase steadily in 

 thickness. The overburden of neve is arranged in regular bands, 

 each of which corresponds to a single year's addition. This being 

 so, it is possible to make some sort of an estimate of the age of the 

 formation. 



The weight of these additions depresses the top of the original 

 ice below the surface of the water. Though there is a regular 

 annual addition above, it must not be imagined that the total 

 thickness of the pontoon is correspondingly increased; for the 

 solution of the lower surface by the sea has also to be reckoned 

 with. Yeiy often, however, in the neve sections of glacier-tongues 

 the cliff-face above the water is observed to stand higher than in the 

 wholly ice zone at the land end. This is to be expected on account 

 of the lighter nature of the neve ice added, there being a larger 

 proportion of air sealed up in it. 



The observed height above sea-level of Antartic shelf-ice so far 

 recorded ranges from about 20 to over 200 feet. A common figure is 

 from 90 to 120 feet, suggesting a total thickness of 600 to 1,000 feet. 



Although the height of the cliif-face presented by shelf-ice gives 

 some idea of its total thickness, a really accurate method of 

 determination is badly needed. The Australasian Expedition hit 

 upon a method which gives positive results in some cases at least. 

 This consists in taking serial temperatures of the sea-water in depth 

 near the face of the shelf-ice. As there is always a current flowing 

 beneath the ice, the bottom of it is likely to be marked by a sudden 

 slight change in the water temperature, easily observed when the 

 observations are plotted as a graph. 



The President conveyed the thanks of the Society to Sir Douglas 

 Mawson for his luminous description of Antarctic conditions and for 

 his selection of the magnificent illustrative photographs on this as 

 well as on a former occasion. The fact that the explorer was in this 

 case a thoi'oughly competent geologist was indeed fortunate. The 

 Fellows had been thus enabled to participate without effort in the 

 new knowledge gained through heroic labour by the lecturer and his 

 comrades. A further privilege was afforded to the Glacial geologists 

 present by Sir Douglas Mawson's readiness to impart the information 

 that he, more than any other man, possessed, and, as time was 

 limited, the President hoped that the speakers would take advantage 

 of this privilege rather than give expression to their particular views. 



