-32 THE POPULAR SCIENCE MONTHLY. 



and portions wore rent off and floated away. If tliis be the true history of the 

 formation of these icebergs, the absence of all land debris in the portion exposed 

 above the surface of the sea is readily understood. If any such exist, it must be 

 confined to the lower part of the berg, to that part which has at one time or 

 other moved on the floor of the ice-cap. 



"The icebergs, when they ai"e first dispersed, float in from 200 to 250 fath- 

 oms. When, therefoi'e, they have been drifted to latitudes of 65 or 64 south, 

 the bottom of the berg just reaches the layer at which the temperature of the 

 water is distinctly rising, and it is rapidly melted, and the mud and the pebbles 

 with which it is more or less charged are precipitated. That this preci[)itation 

 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 entire- 

 ly composed of such deposits; for the diatoms, Glohigerinw, 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 tlie south of the fiftieth parallel, 

 as far as 80 south, 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 Avith 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 corresponding 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 tlie 

 shores of the Mediterranean, Avhich is made up of similar materials. 

 Moreover, in the case of fresh-water deposits of this kind, it is cer- 

 tain that the action of percolating water may convert the originally 

 soft and friable, fine-grained sandstone into a dense semi-transparent 

 opaline stone, the silicious organized skeletons' being dissolved, and 

 the silex redeposited in an amorphous state. Whetlier such a meta- 

 morphosis as this occurs in submarine deposits, as well as in those 

 formed in fresh water, does not appear; but there seems no reason to 

 doubt that it may. And hence it may not be hazardous to conclude 

 that very ordinary metamorphic agencies may convert these polar 

 caps into a form of quartzite. 



In the gi'eat intermediate zone, occupying some 110 of latitude, 

 which separates the circurapolar arctic and antarctic areas of silicious 

 deposit, the diatoms and RacUolaria of the surface-water and the 

 sponges of the bottom do not die out, and, so far as some forms are 

 concerned, do not even appear to diminish in total number ; though, 

 on a rough estimate, it would appear that the proportion of Madiola- 



