Ill EXPEDITION OF THE "CHALLENGER" 81 



rising, and it is rapidly melted, and the mud and pebbles with 

 which it is more or less charged are precipitated. That this 

 precipitation takes place all over the area where the icebergs are 

 breaking up, constantly, and to a considerable extent, is evident 

 from the fact of the soundings being entirely composed of such 

 deposits ; for the Diatoms, Globigerince, and radiolarians are 

 present on the surface in large numbers ; and unless the deposit 

 from the ice were abundant it would soon be covered and 

 masked by a layer of the exuvia of surface organisms." 



The observations which have been detailed 

 leave no doubt that the Antarctic sea bottom, 

 from a little to the south of the fiftieth parallel, 

 as far as 80 S., is being covered by a fine deposit of 

 silicious mud, more or less mixed, in some parts, 

 with the ice -borne debris of polar lands and with 

 the ejections of volcanoes. The silicious particles 

 which constitute this mud, are derived, in part, 

 from the diatomaceous plants and radiolarian 

 animals which throng the surface, and, in part, 

 from the spicula of sponges which live at the 

 bottom. The evidence respecting the correspond- 

 ing Arctic area is less complete, but it is sufficient to 

 justify the conclusion that an essentially similar 

 silicious cap is being formed around the northern 

 pole. 



There is no doubt that the constituent particles 

 of this mud may agglomerate into a dense rock, 

 such as that formed at Oran, on the shores of the 

 Mediterranean, which is made up of similar 

 materials. Moreover, in the case of freshwater 

 deposits of this kind, it is certain that the action 



192 



