62 MR. F. DEBEXHAM OX A XEAV [vol. lxXV, 



the mound of marine mud does not lie in a direct line with either 

 the David or the Larsen glaciers, but somewhat to the side, as in 

 an ice-eddy or ' backwater.' 



An ingenious alternative is suggested by Prof. David & 

 Mr. Priestley, which has chiefly in view the explanation of the 

 •organisms being in a position of growth. The portion of the ice- 

 sheet covered with moraine is supposed to have become overloaded 

 with englacial boulders, until its mean density exceeded that of 

 water. It would then sink, and for a time could form a temporary 

 sea-floor for the lodgment of the muds on its surface. The chief 

 objection to such an hypothesis is that, in order to increase the 

 •density of ice to that extent, the proportion of included rock 

 would have to be about 1 : 3 throughout the mass, a proportion 

 which is never found in the land-ice of Antarctica. 



As the above-mentioned writers point out, to call in any exten- 

 sive recent elevations or depressions of the land as an explanation 

 is not warranted by other evidence. There are some true raised 

 pebble-beaches and terrace-lines along this coast, and there is no 

 doubt that there have been recent upward movements ; but they 

 are limited to some 80 feet or so, and in any case would not explain 

 the occurrence of the muds on floating ice. 



In their account, Prof. David & Mr. Priestley do not include 

 the mirabilite as among the uplifted deposits, but consider it as a 

 concentration from a saline lake. 



A complete explanation of these phenomena is, therefore, still 

 wanting, and in view of the number of these occurrences now 

 known it is desirable that the problem should be solved. Any 

 agent which can reverse the usual order of things and transport 

 material upwards as well as horizontally is well worth studying, and 

 therefore no apology seems necessary for this detailed treatment. 

 In my view the agency at work is no new one. but rather an old 

 one which has been somewhat neglected bv modern sreoloffisfcs. The 

 proof of this will lead into an apparent digression at this point, 

 for it is necessary to describe rather fully the ice-conditions of 

 McMurdo Sound, the type area. 



As will be seen in the bird's-eye view of the Sound (fig. 2. p. 54), 

 the whole of its southern end is rilled with a sheet of ice which may 

 be regarded as the confluence of two streams: the Koettlitz Glacier 

 coming from the south-west, and the Barrier ice from the south- 

 east. Their union north of Brown Island is marked by a vague 

 shear-crack, showing that there is a slight difference in their rates 

 of movement, and their directions of movement are very clearly 

 indicated by the long lines of moraine streaming northwards to end 

 at the sea in a point usually known as the Pinnacle Ice. The 

 thickness of this sheet varies considerably, and the exact figures 

 are difficult to ascertain except at the edges. Those plotted on the 

 map can only be regarded as approximations, being calculated from 

 various pieces of evidence on the basis of a ratio of 6:1, ice below 

 water to ice above. The only evidence for the thickness at a 

 distance from the edge is a series of heights taken by aneroid, by 



