3G Mr Maclaren on Coral Islands and Reefs, as 



and shells, accumulate till they form a bank rising from six to 

 twelve feet above high water, with the highest side towards 

 the sea, from which the surface slopes inward to the lagoon. 

 The ordinai'y width of these islets is under a quarter of a mile, 

 and their length varies from a few yards to several miles. 



In the above cut, No. 1 is a plan of Keeling Atoll, in S. latitude 12°., 

 and E. longitude 96.64°, the structure of which Mr Darwin examined 

 with peculiar care. 



a, d, b, r, i, t,f, the coral reef; the scale being \ of an inch to the mile, 

 the largest diameter of the atoll is 9 miles, and the shortest 7. 



N, the lagoon, which, a little northward of the centre, is 8 fathoms 

 deep, as marked in the figure. The part south of the dotted line is nearly- 

 dry at low water. 



i, t, the dark space here on the surface of the reef, is a long narrow 

 islet of an irregular figure. There are other two between b and r ; smaller 

 ones at/, d, and a; and others of very minute size between/and t. 



There is a wide breach in the reef between b and d, and a narrower one 

 between d and a, either of which admits a ship. 



