28 AUSTRALASIA. 



break with a crash, and the scattered fragments of the crystalline mountains lose 

 that tabular form which is so characteristic of the southern as compared with the 

 northern icebergs. Gradually breaking into smaller pieces, the débris floats away 

 in long convoys, where it is no longer possible to distinguish those of marine from 

 those of glacier origin. 



According to the quantity of the drifting ice and the velocity of the currents 

 the fragments advance to a greater or lesser distance northwards, as a rule, 

 however, seldom penetrating much beyond the 55° of south latitude. Yet they 

 have not imfrequently been met much nearer the equator, especially to the 

 west of New Zealand and in the South Atlantic, where they have been seen as 

 far north as Tristâo da Cunha, and off the Cape of Good Hope under the thirty- 

 fourth parallel. On an average the austral advance 240 miles nearer to the 

 equator than the northern icebergs. The largest observed by the Challenger 

 was about 250 feet high ; but Cook recorded one over 330 feet, while several 

 full}^ one-third higher were measured by Wilkes. They range as a rule from 

 1,500 to 3,000 feet in breadth, yet none of those seen by the naturalists of the 

 Challenger carried any fragments detached from the rocky mountain slopes, 

 although such cases were frequently observed by Ross, Dumout d'Urville, and 

 other explorers. A sketch by John MacNab, who accompanied Balleny's ex- 

 pedition of 1839, represents an iceberg bearing a black rock embedded between 

 two cr> stal nippers. Another huge mass seen by Weddell was so covered with 

 blackish clay that at a distance it would certainly have been taken for a cliff. 



Volcanic; Agencies. 



Drift ice thus contributes in some measure to modify the form of the continents 

 by transporting débris of all kinds to the islands scattered for thousands of miles 

 over the ocean, or depositing them on the marine bed and in this way perhaps 

 laying the foundation for future barrier reefs. But other ageacies are also at 

 work, in one place enlarging, in another diminishing the contours of the oceanic 

 lands. The researches of naturalists have shown that during the course of long 

 ages these agencies have accomplished considerable changes in the geography of 

 the Pacific islands. In the work of modification the chief part has been played 

 by the submarine igneous forces, and the coralline " island builders," which strew 

 the seas with their marvellous structures. 



Volcanoes are far more numerous and energetic in the Pacific basin and sur- 

 rounding continental seaboards than on the opposite shores of the Old and New 

 World washed by the xVtlantic. The fires of Iceland, the Azores, the Canaries, the 

 Cape Verd Islands and AVest Indies, pale before those which follow at intervals 

 around the vast semicircle formed by the coasts of the mainlands sweeping round 

 from Cape Horn to the Cape of Good Hope. The craters are reckoned by hundreds 

 in this " fiery circle " some 20,000 miles in extent, which reaches from the northern 

 island of New Zealand to the southern shores of Chili. Here the chain of burn- 

 ing mountains, occasionally interrupted by wide intervals, especially north of New 



