part 2] MODE OF TBANSPOBTATION BY ICE. G7 



The case of the Lower Ferrar Glacier is precisely similar. It is 

 afloat, and its movement is very slow ; it is subject to excessive 

 thaw, and it is in so deep an inlet that there can be very little 

 movement of the water on which it is resting. The conditions 

 for decrease from above and increase by freezing from below are 

 even more favourable here. 



About the Nansen ice-sheet much less is known, but there again 

 the general conditions are favourable. It is sheltered from the 

 coastal currents, and is not very thick. At the points where the 

 raised muds have been found the movement is certainly slow, and 

 the thaw is great. 



Probably the freezing on the lower surface of these sheets is not 

 general : for instance, it would not take place close to the sea-edge 

 where the movement of water would be appreciable, or, if it did so 

 during the winter, it would disappear during the next summer. 

 But far under the sheet, Avhere the water is comparatively still, 

 there is probably an annual increase from below which will be 

 inversely proportional to the thickness of the ice and directly pro- 

 portional to the stagnation of the water. 



The origin of the raised marine muds now becomes fairly obvious, 

 and may be described in this way, for the McMurdo Sheet. The 

 overflow from the Barrier moving slowly northwards through the 

 volcanic islands is subject to the two processes, decrease by thaw 

 from above and addition by freezing from below. The rates of 

 both these processes would vary, particularly the thaw, for it is 

 now well known that the weather varies great!}" from year to vear, 

 and there is possibly a cyclic variation as well. The movements 

 of the ice in a vertical direction would then be of the following 

 nature. A season, or a run of seasons, of heavy thaws and warmer 

 winters would have the effect of decreasing the total thickness of the 

 sheet and lifting it off the bottom wherever it had been touching. 

 On the other hand, a series of colder seasons would thicken the 

 mass, and it would not only rest upon an increased area of the 

 bottom, but would freeze on to some of the muds and enclose them. 

 These would be lifted when next the mass floated at that point, 

 and in the course of many years would finally appear at the upper 

 surface, having been enclosed by ice and therefore perfectlv pre- 

 served on their way up. 



The case of the mirabilite is a little more complicated, but follows 

 from the same conditions. The bottom of the sound, under the 

 ice, is certainly not level, and may be supposed to contain basins. 

 When the ice-sheet rests on the bottom it will enclose a certain 

 amount of water in each of these basins, cutting it off from the 

 rest of the sea. Ice will continue to be deposited from the water, 

 and it will become increasingly concentrated into a brine solution. 

 This happens in all the small lakes on Cape Hoyds, as mentioned 

 by Prof. David & Mr. Priestley, who found the temperature of the 

 liquid brine residue as low as —17° Fahrenheit. At a certain 

 concentration and temperature the sodium sulphate in the brine 

 would be deposited as solids. The precipitation of this salt from 



Q. J. G. S. No. 298. g 



