108 MARION EXPEDITION TO DAVIS STRAIT AND BAFFIN BAY 



region is estimated to have a volume of oO.OOO.OOO cubic feet, while 

 the average for the Grand Bank is about G,UOU,()00 to 8,00;),000 cubici 

 feet. The average being around Newfoundland is about 100 feet 

 high. The average berg in Disko Bay was estimated by the Marion^ 

 expedition to be twice as high as the average Grand Bank's berg} 

 but it is very difficult to judge accurately, because the height of the 

 land surrounding Disko Bay dwarfs them. The general form of thel 

 Disko bergs api)ears bulkier, and there are few of the deep-valleyed| 

 forms that are so connuon around Newfoundland. The usual melt- 

 ing signs which also appear on the underwater surface of the bergsj 

 in lower latitudes are less apparent in Disko Bay. 



Structure, Color, and Density of Icebergs 



It is well known that the ice composing icebergs is formed from 

 snow crystals which have gradually sublimated into large granular 

 ice. This process of ice formation entangles and imprisons a vast 

 quantity of air in the mass producing a structure which is quite 

 different from ordinary pure ice. Barnes (190(3) shows that the 

 material is relatively hard below temperatures of 15° to 20° F.. but 

 if it be warmed, its power of resistance rapidly decreases, giving 

 away almost entirely at the melting point, wiiere it is soft indeed. 

 An iceberg which invades the Atlantic and comes under summer tem- 

 peratures, becomes pudgy and soft on its outside. A projectile 

 plunges into the berg with a " chug " but in cold weather it only 

 shakes down a shower of dustlike crystals. Steenstrup (18S3) ob- 

 served that by reducing the ten;perature of berg samples the air 

 bubbles inclosed in the ice were subjected to a strong comi)ression,i 

 and in that condition ])ricking by a needle Avas sufficient to burst the 

 ice with the crack of an explosion. Barnes (192Ta, p. 93). however, 

 from his observations upon the size of the air bubbles in the ice, and 

 after they have been released, claims that under natural conditions 

 the air imprisoned in iceberg ice is under no greater than atmospheric 

 pressure. It is also interesting to note that Barnes found that the 

 air imprisoned in iceberg ice probably thousands of years ago has 

 the same composition as the atmosphere of the present day. lie has 

 published the results of some recent experiments (Barnes 1928; 

 ]). 345) to determine the amount of air contained in berg ice, and 

 although the work is preliminary, it was found that 7 to 15 per cent 

 with an average of 10 per cent of the co-volume was air.^^ Small 

 fragments of iceberg ice floating in sea water have been observed ta 

 effervesce with a s))uttering sound ])lainly audible a foot distant quite 

 similar to the frying of bacon. Such a reac-tion would ]^r()l)al)ly indi- 

 cate furthermore the release of innumerable minute air bul)ble-; under 

 greater than atmosi)heric j)ressure. Lieutenant Gonnnander Ricketts 

 informs me that he has observed large bubbles of air rising to thei 

 sea surface on calm days near the sides of icebergs. I^ndoubtedly the' 

 melting of the ice under water causes many of the small bubbles toi 

 collect and arrive at the sea surface as large aii" bubbles. The -ize of 

 the bubbles jjernu'ating the ice may vary considerably froui a diam-i 

 eter of one-fourth inch or more to a nunuteness invisible to the nakedi 

 eye. The average is less than the size of the head of a connr.on pin. 



ss Thunis (1014, p. 08) found a rontont of 1 per cent, but, he adds, the ice used for the 

 experiments was uniistiallv clear and prohaUlv contained a ininininin amount of air. 



