290 A NATURALIST ON THE " CHALLENGER." 



and 10 in extreme breadth. The island is entirely composed of 

 coral-reef rock, without, as far as is known, any blown-sand 

 formation. The sand on the beaches is scanty. The presence 

 of blown sand-rock on coral islands must depend on the freedom 

 of some part of the coast from breakwaters of coral, in order that 

 a heavy surf may form sand in abundance. In Bermuda the 

 sand is derived from the unsheltered side of the island. 



In some rock, about 30 feet above sea level, I saw, as Dana 

 describes, some Brain Corals imbedded in the position in which 

 they had grown. About the reefs are to be seen curious cylin- 

 drical blocks of coral standing on end, and often hollowed out at 

 the top. These arise from the growing of a mass of ordinarily 

 rounded coral until the top reaches the surface of the water or 

 an insufficient depth to allow of further growth. The top of 

 the mass then dies, whilst growth goes on at the sides, and 

 the dead core is hollowed out by decay. 



The surface of the rock in Tonga is covered with a reddish 

 soil, like that of Bermuda. It is so hidden with soil and vege- 

 tation that it is very difficult to observe the rock structure. 

 The wells, round holes sunk to a depth of four or five feet close 

 to the shore, show a mere continuation of the reef-structure of 

 the shore covered by about a foot of soil. 



I w T as interested to recognise amongst the littoral plants of 

 Tonga, many forms which I had gathered on the shores of the 

 far-distant Bermuda. They were cosmopolitan tropical plants, 

 and became familiar objects on nearly all the tropical shores 

 visited subsequently. One plant grows in Tonga which is 

 almost identical with one occurring in Kerguelen's Land, but 

 it again is cosmopolitan, and a w T ater weed, Nitella flexilis. To 

 remind one of Australia, there are Casuarina trees in Tonga, but 

 they are nowhere abundant. 



In every direction in Tonga are large tracts of land which 

 have been under cultivation, but are now overrun with a wild 

 growth, affording plain evidence of the reduction of the popu- 

 lation. These tracts are overrun with a dense low tangle of 

 several species of convolvulus and a trailing bean. The position 



Communications by Mr. W. Mariner, severa years resident in those 

 Islands." By John Martin, M.D., London, 181 7. 



