described by Mr Darwin. 37 



The island abounds in cocoa trees,' sprung from nuts brought 

 by the currents of the ocean from Sumatra or Java, 600 miles 

 distant. Turtles browse on the sea-weeds which grow in the 

 lagoon. The islands are inhabited, and these two articles sup- 

 ply the people with food. What is singular, fresh water is ob- 

 tained from wells which ebb and flow with the tides. Mr Dar- 

 win thinks that the rain water being specifically lighter than 

 the salt, keeps floating on its surface, and is subject to the 

 same movements. 



Barrier Reefs. — Besides the atolls, which have merely a sheet 

 of water in the interior, there are many reefs in the Pacific 

 and Indian Oceans which encircle one or more islands of pri- 

 mary, secondary, or volcanic rock. To these Mr Darwin gives 

 the name of " barrier reefs," and the water which separates the 

 islands from the reef is called " the lagoon channel." These 

 reefs resemble the others inall respects. They support scattered 

 lineal islets ; they are pierced by breaches ; their exterior sides 

 are steep and deep, while their interior are shallow and slope 

 gently. Fig. 2. represents one of these (Maurua) on the same 

 scale as the last. 



r, /, the reef, with two long narrow islets at its northern end, and some 

 smaller ones at other parts. 



N, the lagoon channel. The narrow entrance on its south side ha 

 from four to five fathoms of water. 



L, an island 2 miles long, and 800 feet high in the lagoon. 



In this instance, the lagoon channel, separating the island 

 from the reef, is of small depth and narrow, the breadth rang- 

 ing from a furlong to a mile ; but in other cases, it is 20 miles 

 broad and GO fathoms deep ; and, instead of one or two islands, 

 almost filling the lagoon (as at Raiatea), there are sometimes 

 four, six, or more, of small size, forming mere spots in it. This 

 is exemplified at Hogoleu and Gambler Islands. There are 

 two very remarkable barrier reefs known. The first is that 

 which runs along the north-east coast of Australia 1000 miles 

 in length. It is divided from the land by a lagoon channel 

 from 10 to 30 miles broad, and from 10 to CO fathoms deep. 

 The other runs parallel to the shores of New Caledonia for a 

 length of 400 miles. It accompanies the shores for 250 miles, 

 and continues for 150 miles more in the same direction, afford- 

 ing presumptive evidence that the island has a submarine pro- 



