136 THEORY OF THE FORMATION Ch. V. 



reefs ought to graduate into each other. Eeefs having 

 an intermediate character between those of the fring- 

 ing and barrier classes do exist ; for instance, on the 

 S.W. coast of Madagascar, a reef extends for several 

 miles, within which there is a broad channel from 

 7 to 8 fathoms deep, but the sea does not deepen 

 abruptly outside the reef. Such cases, however, are 

 open to doubt, for an old fringing-reef which had 

 extended itself on a basis of its own formation, would 

 hardly be distinguishable from a barrier-reef produced 

 by a small amount of subsidence, and with its lagoon- 

 channel nearly filled up with sediment during a long 

 stationary period. Between barrier-reefs, encircling 

 either a single lofty island or several small low ones, 

 and atolls including a mere expanse of water, a striking 

 series can be shown ; and in proof of this, I need only 

 refer to Plate I., which speaks more plainly to the eye, 

 than any description to the ear. The authorities from 

 which the figures have been copied, together with some 

 remarks on them, are given on a separate page descrip- 

 tive of the plate. At New Caledonia (Plate II. fig. o) 

 the barrier-reefs extend for 150 miles on each side of 

 the submarine prolongation of the island ; and at the 

 northern extremity these reefs appear broken up and 

 converted into a vast atoll supporting a few low coral- 

 islets. We may imagine that we see in New Caledonia 

 the effects of subsidence actually in progress, — the 

 water always encroaching on the northern end of the 

 island, towards which the mountains slope down, and 

 the reefs steadily building up their massive fabrics in 

 the line of their ancient growth. 



