io6 



NA TURE 



[Nov. 30, 1876 



the blue lines have lost their dead whiteness and have 

 become hyaline or bluish. The blue lines are very un- 

 equal in their strength and in their depth of colouring ; 

 sometimes a group of very dark lines gives a marked 

 character to a part of a berg. Between the stronger blue 

 lines near the top of the cliff a system of closerbands 

 may be observed, marking the division of the ice by 

 still finer planes of lamination, but these are blended and 

 lost in the narrower spaces towards the water-line. The 

 blue lines are the sections of sheets of clear ice; the 



Fig. I. — February 11, 1874. Lat. 60" 52' S , Long. 80° 20' E. 



white intervening bands are the sections of layers of 

 ice where the particles are not in such close contact— ice 

 probably containing some air. 



The stratification in ;dlthe icebergs which we saw, was, 

 I believe, originally horizontal and conformable, or very 

 nearly so. In very many of them the strata had become 

 inclined at various angles, or vertical, or reversed ; in 

 many they were traversed by faults, or twisted or con- 

 torted or displaced, but I never saw a sirg'e instance of 

 deviation from the horizontal and symmetrical stratifica- 

 tion which could in any way be referred to original struc- 

 ture ; which could not, in fact, be at once accounted for 

 by changes which we had an opportunity of observing 

 taking place in the icebergs. 



I think there can be no reasonable doubt from their 

 shape, and from their remarkable uniformity of character, 



Fig. 2.— Febniarj 23, i?74. L?.t. 64° 15' S., Long. 93° 24' E. 



that these great table-topped icebergs which we saw all 

 around us, and closing in our southern and eastern horizon 

 at the southernmost point of our voyage, and breaking 

 down and melting a little further to the north, are pris- 

 matic blocks riven from the edge of the great Antarctic 

 ice-sheet, portions of whose vertical cliff were seen by 

 Ross in January, 1841, and in February, 1842, in lat. 

 78° 4' and lat. jf 49' S. to the southward of New Zealand, 

 and by Lieut.-Commandant Ringgold in the U.S. ship 

 Porpoise, on February i and 2, 1840, in long. 130° 36' E. 



lat. 65° 49' S. There is unfortunately great difficulty in 

 determining when the wall of ice to which the term " ice- 

 barrier " was restricted by Capt. Ross was seen by- 

 Lieut. Wilkes or any of his party, since Lieut. Wilkes 

 applies the term indiscriminately to the solid ice-walls 

 and to the masses of moving pack by v.'hich his progress 

 was from time to time interrupted. The wall is satisfac- 

 torily described at one or two points only. 



I iaelieve that the stratification of those portions of the 

 icebergs which were visible to us is due entirely to suc- 

 cessive accumulations of snow upon a nearly 

 level surface. The spaces between the trans- 

 verse blue lines on the bergs may possibly 

 represent approximately the snow accumula- 

 tion of successive seasons. The direct radiant 

 heat of the sun is very great in these latitudes, 

 and during summer the immediate surface of 

 the snow is frequently melted in the middle of 

 the day, the water percolating down among 

 the snow beneath and freezing again at night, 

 or when it has trickled down into the shade. 

 This process repeated every clear day for the 

 two or three months of summer might well 

 convert a very considerable belt of snow into 

 ice more or less compact. That the process does go on 

 we had ample evidence in the icicles fringing the snow 

 which was lying upon flat pieces of the pack, which 

 dropped rapidly in the sun even when the thermometer 

 in the shade was several degrees below the freezing 

 point. 



The finer laminations may probably indicate the more 

 feeble results of the same process after successive snow- 

 falls. As I have already said there was not, so far as we 

 could see, in any iceberg the slightest trace of structure 

 stamped upon the ice in passing down a valley, or during 

 its progress over roches montonnees or any other form of 

 uneven land ; the only structure except the parallel strati- 

 fication which we ever observed which could be regarded 

 as bearing upon the mode of original formation of the 

 ice-mass was an occasional local thinning out of some of 

 the layers and thickening of others, just such 

 an appearance as might be expected to result 

 from the occasional drifting of large btds of 

 snow before they have time to become con- 

 solidated. 



We certainly never saw any trace of gravel 

 or stones or any foreign matter necessarily 

 derived from land on an iceberg; several 

 showed vertical 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 

 chiefly due to the foreign material filling the 

 fissure reflecting Ight le^s perfectly than the 

 general surface of the berg. In one or two 

 distant bergs there seemed to be thick hori- 

 zontal beds of ice deeply coloured brown or 

 -'':- bottle-green, but this was also, I believe, chiefly 

 an effect of light. 



In the pack, which is made up of fragments 



of all sizes of berg-ice mixed with masses of 



salt-water ice, the berg-ice is almost always 



either white with pale- blue streaks, blue with 



a white opalescence, or rarely deep blue, or 



still more rarely black from absolute transparency ; it is 



seldom soiled in any way. It is so occasionally ; on the 



loth we passed, not far from our turning-point, a piece of 



berg-ice with a small flock of penguins upon it. The 



birds had evidently been there for some time for the snow 



on the surface of the ice was trampled into a dirty brownish 



mud ; another fall oi snow would have converted this 



layer into a discoloured vein in the block. 



C. Wyville Thomson 

 {Xo ^<? continued.') 



