122 



NATURE 



\Dec. 7, 1876 



It is not easy to see why the temperature of the earth's 

 crust under a widely extended and practically permanent 

 ice-sheet of great thickness should ever fall below the 

 freezing-point ; and it is matter of observation that at all 

 seasons of the year vast rivers of muddy water flow into 

 the frozen sea from beneath the great glaciers which are 

 the issues of the ice-sheet of Greenland. Ice is a very 

 bad conductor, so that the cold of winter cannot penetrate 



to any great depth into the mass. The normal tempera- 

 ture of the crust of the earth at any point where it is 

 uninfluenced by cyclical changes is, at all events, above 

 the freezing-point ; so that the temperature of the floor of 

 the ice-sheet would certainly have no tendency to fall 

 below that of the stream which was passing over it. The 

 pressure upon the deeper beds of the ice must be enor- 

 mous ; at the bottom of an ice-sheet 1,400 feet in thick- 





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Fig. 4. — Iceb:-rg pa'sed on February 21, ib;4 i^at 



c9 



ness it cannot be much less than a quarter of a ton on the 

 square inch. It seems therefore probable that under 

 tVe pressure to which the body of ice is subjected a 

 constant system of melting and regelation may be taking 

 place, the water passing down by gravitation from layer 

 to layer until it reaches the floor of the ice-sheet, and 

 finally working out channels for itself between the ice 

 and the land whether the latter be sub- aerial or sub- 

 merged. 



I should think it orobable that this pro- 

 cess or some modification of it may be the 

 provision by which the indefinite accumu- 

 la ion of ice ovc:r the vast nearly level '^ 



regions of the "Antarctic continent" is °'" 



prevented, and the uniformity in the thick- 

 ness of the ice- sheet maintained ; that in 

 fact ice at the temperature at which it is in "^J 

 contact with the surface of the earth's crust * 

 within the Antarctic regions cannot support 

 a column of itself more than 1,400 feet high 

 without melting. 



When the icebergs are drifted in the 

 summer a little to the northward — in the 

 meridian of 80° E. to the parallel of 64° S. — 

 they begin to disintegrate very rapidly. The 

 water at the surface of the se i rises to zero 

 and slightly above it, and dashing against 

 the windward side of the berg, partly by its 

 mechanical action, but more by the constant 

 and rapid renewal of the warm water, it 

 soon wears a deep groove in the face of 

 the clifl". When the groove has cut in so far that cohesion 

 will no longer maintain the weight of unsupported ice, 

 ^hich seems usually to be the case when it is 10 to 15 

 feet deep, a mass of the cliff falls down, and the weight 

 of the berg being reduced on that side it tilts up more or 

 less and assumes an inclmed position ; the stratification 

 thus becomes inclined, although it still remains conform- 

 able with the plane of the top of the iceberg. 



The sea now washes up on the low portion which has 

 been exposed by the tilting of the b^rg, which it soon 



reduces to a beautiful curved slope to the bottom of the 

 new cliff, and the process is repeated until by re^jeaied 

 falls of the face of the cliff one side of the berg is so much 

 lightened, that the preponderating weight of the opposite 

 side raises the newly exposed portion out of the water ; 

 giving the berg a double outline and the veining a high 

 inclination. 



We frequently saw table- topped icebergs with the upper 

 sur'ac-^ very irregular ; when that is the case evidence 



G 5.— February 25, 1874. Lat. 63° 49' S , Long. 94° 51' £. 



may usually be found from the colour, the closeness of the 

 veining, and other appearances, that it is not the original 

 surface of the iceberg which is now presented to us, but 

 a second surface produced by the cutting away by the 

 sea of an entire story, as it were, of the berg; which 

 athough it had no doubt at one time during the process 

 been greatly inclined, had recovered its equilibrium on the 

 whole of the upper layer having been more or less S)m- 

 metrically removed. Fig. 3 is a view of an iceberg in 

 which the whole of the upper tier seems to be breaking 



