CORAL-REEFS. 105 



less than ten years, in the lagoon of Keeling atoll, should 

 be here borne in mind. We may also infer, from the 

 trouble which the inhabitants of the Maldiva atolls 

 take to root out, as they express it, the coral-knolls from 

 their harbours, that their growth can hardly be very 

 slow. 1 



From the facts given in this section, it may be con- 

 cluded, first, that considerable thicknesses of rock have 

 certainly been formed within the present geological sera by 

 the growth of coral and the accumulation of its detritus ; 

 and, secondly, that the increase of individual corals and 



1 Mr. Stutchbury {West of England Journal, No. I, p. 50) has 

 described a specimen of Agaricia, "weighing 2 lbs. 9 oz., which 

 surrounds a species of oyster, whose age could not be more than two 

 years, and yet is completely enveloped by this dense coral." I pre- 

 sume that the oyster was living when the specimen was procured ; 

 otherwise the fact tells nothing. Mr. Stutchbury also mentions an 

 anchor, which had become entirely encrusted with coral in fifty years ; 

 other cases, however, are recorded of anchors which have long 

 remained amidst coral-reefs without having become coated. The 

 anchor of the Beagle, in 1832, after having been down exactly one 

 month at Rio de Janeiro, was so thickly coated by two species of 

 Tubularia, that large spaces of the iron were entirely concealed ; the 

 tufts of this horny zoophyte were between two and three inches in 

 length. It has been attempted to compute, but I believe erroneously, 

 the rate of growth of a reef, from the fact mentioned by Capt. Beechey, 

 of the Chama gigas being embedded in coral-rock. But it should be 

 remembered, that some species of this genus invariably live, both 

 whilst young and old, in cavities, which the animal has the power of 

 enlarging with its growth. I saw many of these shells thus embedded 

 in the outer ' flat ' of Keeling atoll, which is composed of dead rock ; 

 and therefore the cavities in this case had no relation whatever with 

 the growth of coral. M. Lesson, also, speaking of this shell (Partie 

 Zoolog., Voyage de la Coquille), has remarked, "que constam- 

 ment ses valves etaient engages completement dans la masse des 

 Madrepores." 



