OF THE SCOTTISH NATIONAL ANTARCTIC EXPEDITION. 681 



of deposition of rock debris will not explain their absence. To account for them being 

 so completely lacking until the Diatom ooze belt is reached, it seems necessary to 

 postulate that they sink more slowly than even the minutest mineral particles and 

 clayey matter constituting such a preponderating part of the glacial clays furthest 

 from the inland ice. 



Another possibility is that the rock flour is liberated at such a depth below the 

 surface so as never to come to the same extent as the diatoms under the influence of 

 the north-going current which carries them off". It therefore sinks to the bottom before 

 the diatoms do. 



Ago-lutinative foraminifera with a siliceous cement are common, and where a 

 large amount of material was available from the trawl they were often obtained in 

 large numbers. 



Minerals: (I) Rocks. — The trawl frequently brought up large quantities of rock 

 fragments varying in size from fine gravel up to boulders weighing two cwts. or more. 

 The larger boulders are mostly sub-angular in shape, exactly like those which occur in 

 boulder clays, and they frequently show glacial striae; the smaller fragments may be 

 either rounded or angular. A number of the rock specimens have part of their surface 

 clean and fresh, and part coated with a brownish stain of manganese or iron oxide ; the 

 shape indicates that the latter part has been embedded in the bottom mud, while the 

 former has projected into the overlying bottom water. The association at one spot, 

 along with fine-grained mud or clay, of rock fragments which, according to their size, 

 if assembled together would be called boulders, shingle, gravel, or sand, is one of the 

 characteristic features of the glacial deposits, dependent upon their mode of formation. 



In all other terrigenous deposits, not influenced by ice, the size of the component 

 particles at one place is pretty constant, all lying close to a certain average, which gives 

 the deposit at that place its peculiar character : gravel, sand, mud, etc. But this is 

 not the case with the glacial deposits, for the grain size, when the larger constituents 

 are taken into account as well as the finer, at any particular locality follows no such 

 rule but shows the utmost variation. This is to* be accounted for readily by the mode 

 of formation of these deposits. Most of the material composing them probably comes 

 off the Antarctic land as "ground moraine," along with the inland ice-sheet or glacier 

 tongue protrusions thereof, forming a thick layer or sole to the bergs as they break off: 

 A smaller amount will be frozen in more deeply in the bergs — surface morainic material 

 or ground morainic material which has risen passing some sub-glacial obstruction. As 

 the bergs melt, this material is set free and deposited, the finer-grained debris probably 

 drifting a considerable distance in suspension before it finally settles down on the 

 bottom. Close to the Antarctic shores the glacial deposits are undoubtedly richer in 

 the larger materials, as a great part of the " bottom moraine " is probably liberated very 

 early, perhaps even before the bergs actually calve off the parent ice-sheet ; whilst those 

 deposits far off from land and in deeper water contain proportionately much larger 

 amounts of fine-grained material, but in both situations the intimate admixture of large 



