Volume 14 Australasia - dana ward's homepage

28 AUSTRALASIA.break with a crash, and the scattered fragments of the crystalline mountains losethat tabular form which is so characteristic of the soutliern as compared with thenorthern icebergs. Gradually breaking into smaller pieces, the debris floats awayin long convoys, where it is no longer possible to distinguish those of marine fromthose of glacier oi'igin.According to the quantity of the drifting ice and the velocity of the currentsthe fragments advance to a greater or lesser distance northwards, as a rule,however, seldom penetrating much bej^ond the 55° of south latitude. Yet theyhave not unfrequently been met much nearer the equator, especially to thewest of New Zealand and in the South Atlantic, where they have been seen asfar north as Tristao da Cunha, and off the Cape of Good Hope under the thirtyfourthparallel. On an average the austral advance 240 miles nearer to theequator than the northern icebergs. The largest observed by the Challengerwas about 250 feet high ; but Cook recorded one over 330 feet, while severalfully one-third higher were measured by Wilkes. They range as a rule from1,500 to 3,000 feet in breadth, yet none of those seen by the naturalists of theChallenger carried any fragments detached from the rocky mountain slopes,although such cases were frequently observed by Ross, Dumoiit d'Urville, andother explorers. A sketch by John MacNab, who accompanied Balleny's expeditionof 1839, represents an iceberg bearing a black rock embedded betweentwo cr\ stal nijjpers. Another huge mass seen by Weddell was so covered withblackish claj' that at a distance it would certainly have been taken for a cliff.Volcanic Agencies.Drift ice thus contributes in some measure to raodifj' the form of the continentsby transporting debris of all kinds to the islands scattered for thousands of milesover the ocean, or depositing them on the marine bed and in this way perhapslaying the foundation for future barrier reefs. But other ageucies are also atwork, in one place enlarging, in another diminishing the contours of the oceaniclands. The researches of naturalists have shown that during the course of longages these agencies have accomplished considerable changes in the geograjjhy ofthe Pacific islands. In the work of modification the chief jjart has been playedby the submarine igneous forces, and the coralline " island builders," which strewthe seas with their marvellous structures.Volcanoes are far more numerous and energetic in the Pacific basin and surroundingcontinental seaboards than on the opposite shores of the Old and NewWorld washed by the Atlantic. The fires of Iceland, the Azores, the Canaries, theCape Yerd Islands and West Indies, pale before those which follow at intervalsaround the vast semicircle formed by the coasts of the mainlands sweeping roundfrom Cape Horn to the Cape of Good Hope.The craters are reckoned by hundredsin this "fiery circle " some 20,000 miles in extent, which reaches from the northernisland of New Zealand to the southern shores of Chili. Here the chain of burningmountains, occasionally interrupted by wide intervals, especially north of New