ALEXANDER AGASSIZ 443 
feet above the sea and with a lagoon-basin in the center sunken about 
%0 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, seventy- 
five 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, 
R.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 Gar- 
diner 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 con- 
siderable réle in determining the contours of the atolls, some of them 
being mainly rings of sand-bars enclosing a lagoon, as in the Gilbert 
