30 AUSTRALASIA. 



or are disposed in curves. The Mariannas, the Tonga and Samoan archipelagoes 

 have all their volcanoes, and towards the centre of the circuit of North Pacific 

 burning mountains rises the group of stupendous Hawaiian craters. 



Beyond the circuit towards the Indian Ocean, a formidable igneous chain, 

 beginning to the west of New Guinea, comprises a line of islands west of Timor, 

 Flores, Sumbawa, Sombok, and Bali, together with Java with its forty- five cones, 

 of which twenty-eight are still active. West of Java the volcanic chain no longer 

 runs westwards, but is intersected at a sharp angle by another line of fracture 

 traversing Sumatra with its sixty- seven cones, of which five are still active. On 

 the opposite side of the Indian Ocean rise the insular cones of the Mascarenhas 

 and Comoro group, while Madagascar itself is studded with hundreds of extinct 

 craters. Others, such as those of St. Paul and Amsterdam, follow in the austral 

 waters, here rising amid the surrounding ice floes. 



New Zealand, the Sunda Islands, Japan, the Kuriles and Hawaii are amongst 

 the regions that have been most profoundly modified by igneous agencies, at least 

 during the historic period. But the most active centre on the surface of the globe 

 is probably the Sunda Strait, which marks the precise sjjot where the two volcanic 

 axes of Java and Sumatra intersect each other on the edge of the submarine bank 

 separating the Sunda plateau from the deep abysses of the Indian Ocean. Here 

 is situated the famous island of Krakatau, which lost two-thirds of its area during 

 the eruption of 1883, when other islands rose to the surface, and the atmosphere 

 became charged with volcanic dust wafted by the winds round the periphery of 

 the globe. 



Coralline Formations. — Atolls. 



The changes caused by the coral builders, although accomplished at a much 

 slower rate and without any sudden convulsion of nature, are none the less even 

 move important than those due to igneous agency. In the Pacific alone Dana 

 enumerates two hundred and ninety coralline islands, which with the inner lagoons 

 cover a total area of no less than 20,000 square miles.* If to these be added 

 surfaces large enough to afford space for a village or clump of cocoanut palms, 

 the islands and islets must be reckoned by many thousands which have been 

 constructed by the polypi in the Indian and Pacific Oceans, and especially in 

 the central and western parts of the South Sea. These organisms are unable 

 to carry on their operations in waters whose winter temperature is less than 68° or 

 70° F. But the zone where they find the necessary thermal conditions offers on 

 either side of the equator a variable breadth, in some places exceeding 3,500 

 miles. 



Everywhere within these wide limits, living colonies are able to establish them- 

 selves on the shores and shallows flooded to depths of 130 to 150, and under certain 

 conditions of from 300 to 320 feet. But they are unable to live in waters too 

 highly charged with sedimentary or alluvial matter, and the barrier reefs are con- 



* United States Exploring Expedition, vol. x. 



