But it is in the island world that we must look for the most numerous volcanoes.
A great ridge runs through the Atlantic; and along this lie a number of islands with active centres. Jan Mayen, in the Arctic circle, has an active volcano; Iceland, thirteen; the Azores, six; the Canaries, three; east African islands, eight; the West Indies, six; three submarine volcanoes have been observed at different times in the Atlantic. But through the same region the number of extinct volcanoes is far greater. Of those which exist several seem approaching extinction.
But in the isles of the Pacific and between the Pacific and Indian oceans we have a vast series of volcanic vents of wonderful activity. In the Aleutian Islands are thirty-one; in the Kurile Isles, ten or more; Japan and the adjacent groups have twenty-five. Southeast of the Asiatic continent is the most active region on the globe. Fifty volcanoes are here known. Farther south are four in New Guinea, one or two submarine vents, a number in New Britain, the Solomon group, the New Hebrides, three in New Zealand, and Erebus and Terror in the Antarctic circle. Add to these the islands of the Central Pacific, and we have more than one-half the volcanoes of the globe. Besides there are a large number of nearly perfect volcanic cones which must recently have become extinct.
In conclusion, we find all the oceanic islands are either of volcanic or coral formation; and as we find that the coral polyp can not live at a greater depth than one hundred and twenty feet; as we know the ocean in the immediate neighborhood of these islands to be many thousand of feet in depth; as we know coral islands to be circular, often enclosing a lagoon of water, it is fair to suppose that the polyps have not built through long ages of subsidence, as is usually supposed; but that they have built upon the rims of extinct craters lying near the surface. The fact that these circular reefs always have one or more breaks in their circuit is additional reason for the belief. The fact of a coral island lying within a barrier reef, then,
CORAL REEFS.