24 REPORT— 1881. 



It was in the year 1842 that Darwin published his great work on 

 ' Coral Islands.' The fringing reefs of coral presented no special difBcnlty. 

 They could be obviously accounted for by an elevation of the land, so 

 that the coral which had originally grown under water, had been raised 

 above the sea-level. The circular or oval shape of so many reefs, how- 

 ever, each having a lagoon in the centre, closely surrounded by a deep 

 ocean, and I'isiug but a few feet above the sea-level, had long been a 

 puzzle to the physical geogi'apher. The favourite theory was that these 

 were the summits of submarine volcanoes on which the coral had grown. 

 But as the reef-making coral does not live at greater depths than about 

 twenty-five fathoms, the immense number of these reefs formed an almost 

 insuperable objection to this theory. The Laccadives and Maldives, for 

 instance — meaning literally the 'lac of islands ' and the 'tliousand islands' 

 — are a series of such atolls, and it was impossible to imagine so great a 

 number of craters, all so nearly of the same altitude. Darwin showed, 

 moreover, that so far from the ring of coi'als resting on a corresponding 

 ridge of rock, the lagoons, on the contrary, now occupy the place which 

 was once the highest land. He pointed out that some lagoons, as for 

 instance, that of Vanikoro, contain an island in the middle ; while other 

 islands, such as Tahiti, are surrounded by a margin of smooth water, 

 separated from the ocean by a coral reef. Now, if we suppose that 

 Tahiti were to sink slowly, it would gradually approximate to the con- 

 dition of Vanikoro ; and if Vanikoro gradually sank, the central island 

 would disappear, while on the contrary the gi-owth of the coral might 

 neutralise the subsidence of the reef, so that we should have simply an 

 atoll, with its lagoon. The same considerations explain the origin of the 

 ' barrier reefs,' such as that which runs, for nearly one thousand miles, 

 along tlie north-east coast of Australia. Thus Darwin's theory ex- 

 plained the form and the approximate identity of altitude of these coral 

 islands. But it did more than this, because it showed us that there 

 were great areas in process of subsidence, which, though slow, was of 

 great importance in physical geography.' 



Much information has also been acquired with reference to the 

 abysses of the ocean, especially from the voyages of the Porcupine and the 

 Challenger. The greatest depth yet recorded is near the Ladrone Islands, 

 where a sounding of 4,575 fathoms was obtained. 



Ehrenberg long ago pointed out the similarity of the calcareous mud 

 now accumulating in our recent seas to the chalk, and showed that the 

 green sands of the geologist are largely made up of casts of foraminifera. 

 Clay, however, had been looked on, until the recent expeditions, as 

 essentially a product of the disintegration of older rocks. Not only, 

 however, are a large proportion of siliceous and calcareous rocks either 

 directly or indirectly derived from material which has once formed a 

 portion of living organisms, but Sir Wyville Thomson maintains that 



' I ought to mention that Darwin's views have recently been questioned \>j 

 Semper and Murray, 



