Page:Popular Science Monthly Volume 77.djvu/449

Rh feet above the sea and with a lagoon-basin in the center sunken about 70 feet below the encircling ridge. It is possible, however, that this central concavity may have been formed by solution after the island was raised above the sea, and that the island was not originally an atoll.

The lagoons of the Pacific atolls were found to be usually from 13 to 20 fathoms deep, and to be quite thickly studded with submerged rocks consisting of Tertiary limestone encrusted with modern corals.

The atoll contours are due to a coordination of complex conditions, erosion, currents, silt, etc., which determine the place and rates of growth of the corals; and not to subsidence, as was postulated by Darwin.

The modern coral reefs are, according to Agassiz, distinct from the tertiary limestones, and form a mere crust upon a base of lava or of old limestone.

A notable act of the expedition was the bringing up of the deepest trawl haul ever made, this being from a depth of 4,173 fathoms, seventyfive miles east of Tonga Tabu. Siliceous sponges were found here under an ocean almost as deep as the crests of the Himalayas are high.

In Bora Bora, of the Society group, he found a broken ring of sandy coral islets covered with cocoa palms, and encircling the shallow waters of the lagoon, out of the center of which there arises the towering mass of the basaltic cliffs of the island. The sight of this old volcano, now sleeping and encircled by its palm-crowned atoll ring, so impressed Alexander Agassiz that he employed Mr. G. W. Curtis to make a survey, and to construct a detailed model of the island for the museum at Harvard.

As one goes westward over the tropical Pacific the coral heads upon the reefs become larger and larger, those of the Paumotos being small and stunted, while those of the Great Barrier reef of Australia are the largest in the world.

Alexander Agassiz had now seen nearly all of the coral islands of the Pacific, and he at once turned his attention to the Indian Ocean, cruising among the atolls of the Maldive Islands from December, 1901, to January, 1902. For this purpose he chartered the steamer Amra from the British India Steam Navigation Company, William Pigott, B.N.R., in command. He steamed more than 1,600 miles among the islands, making more than eighty soundings. Mr. J. Stanley Gardiner, M.A., had only recently explored the Maldives, and his account of their mode of formation was published before that of Agassiz. Both Gardiner and Agassiz agree that there is evidence of recent elevation in the Maldives, and that conditions which are operating at the present day are determining the shape of the atolls. Shifting sand-bars play a considerable rôle in determining the contours of the atolls, some of them being mainly rings of sand-bars enclosing a lagoon, as in the Gilbert