38 Mr Maclaren on Coral Islands and Reefs, as 



longation of this extent. At some places it is but a few yards 

 from the island ; at others it is 20 miles ; and so steep was its ex- 

 terior side found to be in one instance, that at two ship-lengths 

 from the reef no bottom was found with a line of 900 feet. 



Double and triple Atolls. — There are small atolls sometimes 

 placed in elliptical rows, with a sheet of water in the centre, 

 and thus becoming constituent parts of a large atoll. This is 

 shewn at fig. 3, where 14 small atolls, each with its little 

 lagoon, are so arranged as to form one large atoll, with a large 

 lagoon, N, in its centre. The figure is ideal, but we have an 

 example in the Maldiva Archipelago, where the combination 

 is carried a stage higher. This group extends over a space of 

 470 miles in length by 50 in breadth, and forms, as it were, 

 three orders of atolls. First, you have a hundred of these 

 little reefs, with pools in the centre, so disposed as to form one 

 large atoll, 50 or 60 miles long, by 10 or 15 broad, with a 

 lagoon 25 fathoms deep. Next, twenty of these large atolls 

 of the second order, are arranged in the shape of a narrow 

 ellipse, so as to form one vast atoll of the third order, 470 

 miles in length by 50 in breadth, with a lagoon in the interior 

 of unfathomable depth. 



The atolls and barrier reefs are dispersed in great numbers 

 over the Pacific and Indian Oceans. Are they the remnants 

 of a former continent which has disappeared, or is disappearing, 

 from that vast watery waste ? — or are they the harbingers of a 

 neio continent which is coming into existence ? These are the 

 questions which Mr Darwin has discussed with great learning 

 and ingenuity. 



Fringing Reefs. — The third form in which coral-reefs pre- 

 sent themselves is, that of Fringing Reefs, the difference be- 

 tween which and the other two must be pointed out. " Atolls" 

 are rings of coral-rock, rising nearly to the surface of the 

 sea, with or without islets of drifted coral generally having a 

 gi-eat depth of water on the outside, and a lagoon from 5 to 

 50 fathoms deep in the centre. " Barrier reefs" are exactly 

 similar, except that they encircle one or more islands of sedi- 

 mentary or volcanic rock, from which they are divided by a 

 lagoon-channel, which, like the lagoons of the atolls, is gene- 

 rally from 5 to 50 fathoms deep. " Fringing reefs" resemble 

 barrier reefs, except that they have a comparatively small 



