80 EXPEDITION OF THE " CHALLENGER '* ni 



the patli of the reflux of the Agiilhas current, than in long. 

 108° E. 



"All along the edge of the ice-pack — everywhere, in fact, to 

 the south of the two stations— on the 11th of February on our 

 southward voyage, and on the 3rd of March on our return, we 

 brought up fine sand and grayish mud, with small pebbles of 

 quartz and felspar, and small fragments of mica-slate, chlorite- 

 slate, clay-slate, gneiss, and granite. This deposit, I have no 

 doubt, was derived from the surface like the others, but in this 

 case by the melting of icebergs and the precipitation of foreign 

 matter contained in the ice. 



"We never saw any trace of gravel or sand, or any material 

 necessarily derived from land, on an iceberg. Several showed 

 Tertical or irregular fissures filled with discoloured ice or snow ; 

 but, when looked at closely, the discoloration proved usually to 

 be very slight, and the effect at a distance was usually due to 

 the foreign material filling the fissure reflecting light less per- 

 fectly than the general surface of the berg. I conceive that 

 the upper surface of one of these great tabular southern ice- 

 bergs, including by far the greater part of its bulk, and culmin- 

 ating in the portion exposed above the surface of the sea, was 

 formed by the piling up of successive layers of snow during the 

 period, amounting perhaps to several centuries, during which 

 the ice-cap was slowly forcing itself over the low land and out 

 to sea over a long extent of gentle slope, until it reached a depth 

 considerably above 200 fathoms, when the lower specific weight 

 of the ice caused an upward strain which at length overcame the 

 cohesion of the mass, and portions were rent off and floated 

 away. If this be the true histoiy of the formation of these 

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

 above tlie surface of the sea is readily understood. If any such 

 exist, it must be confined to the lower part of the berg, to that 

 pai-t which has at one time or other moved on the floor of the 

 ice-cap. 



" The icebergs, when they are first dispersed, float in from 

 200 to 250 fathoms. When, therefore, they have been drifted 

 to latitudes of 65° or 64° S., the bottom of the berg just reaches 

 the layer at which the temperature of the water is distinctly 



