58 CORAL-REEFS. 



But the intermediate portion, lying in the fork of the 

 channel, is bordered by reefs less perfect than those which 

 surround any other atoll in the group of equally small 

 dimensions. Mahlos Mahdoo, therefore, is in every respect 

 in so intermediate a condition, that it may be considered 

 either as a single atoll nearly dissevered into three portions, 

 or as three atolls almost perfect and intimately connected. 

 This is an instance of a very early stage of the apparent 

 disseverment of an atoll, but a still earlier one in many 

 respects is exhibited at Tilla-dou-Matte. In one part of 

 this atoll, the ring-formed reefs stand so far apart from each 

 other, that the inhabitants have given different names to the 

 northern and southern halfs ; nearly all the rings, moreover, 

 are so perfect and stand so separate, and the space from 

 which they rise is so level and unlike a true lagoon, that we 

 can easily imagine the conversion of this one great atoll, 

 not into two or three portions, but into a whole group of 

 miniature atolls. A perfect series such as we have here 

 traced, impresses the mind with an idea of actual change ; 

 and it will hereafter be seen, that the theory of subsidence, 

 with the upward growth of the coral, modified by accidents 

 of probable occurrence, will account for the occasional 

 disseverment of large atolls. 



The Great Chagos Bank alone remains to be described. 

 In the Chagos group there are some ordinary atolls, some 

 annular reefs rising to the surface but without any islets on 

 them, and some atoll-formed banks, either quite submerged, 

 or nearly so. Of the latter, the Great Chagos Bank is much 

 the largest, and differs in its structure from the others : a 

 plan of it is given in Plate IV., Fig. i, in which, for the sake 

 of clearness, I have had the parts under ten fathoms deep 

 finely shaded : an east and west vertical section is given in 

 Fig. 2, in which the vertical scale has been necessarily 



