386 NOTICES OF NEW BOOKS. 



on the degree of inclination of the submarine slope, conjoined 

 with the fact, that reef-building polyps can only exist at limited 

 depths ; so that, where the sea is shallow, they lose their fringing 

 character, and appear as separate and irregularly scattered patches, 

 often of considerable area. These reefs also are generally higher 

 and more perfect on the outer side, assuming the appearance of 

 atolls, owing to the more vigorous growth of the coral where most 

 exposed. They differ, however, from atolls in being less deep, in 

 having their form less defined, and in being based on a shallow 

 foundation. 



Coral reefs are, for the most part, confined to the tropical seas, 

 but extend as far as latitude 32° 15' N. (at the Bermuda islands), 

 and on the western shores of Australia to 29° S. 



They do not seem to be influenced by the presence or absence 

 of volcanic centres, but are totally absent in certain large districts 

 within the tropical seas, and this apparently capricious distribution 

 cannot be explained by any obvious causes. It seems beyond a 

 doubt that the strongest and most massive corals flourish best 

 where most exposed, while the presence of mud and sand, and of 

 brackish or fresh water, is highly injurious. Different species 

 abound in each of the different zones of depth, and in the case of 

 some reefs there is no new material added above the lowest level 

 which the water attains ; although, in others, the growth continues 

 to high water mark. There is also a fact of singular interest and 

 importance with regard to this point in the occasional submerged 

 reefs in the Chagos Bank and elsewhere, which, in some instances, 

 appear to have remained stationary for a very long period, and 

 have no tendency to grow upwards. It is supposed by the author 

 that, in their case the whole group of reefs has subsided some 

 seven or eight fathoms, so that the species building near the sur- 

 face were destroyed, and other species preferring this depth having 

 succeeded to them, there is now little tendency to grow upwards. 

 There is, at any rate, no equal tendency to grow upwards in all 

 coral reefs, but their growth is unequal and dependent on circum- 

 stances. 



There is no doubt that coral, under favourable conditions of 

 growth, increases to an enormous extent, and very rapidly ; and 

 although there are many instances on record of reefs which have 

 not increased for many years, there are others telling a very dif- 

 ferent tale. The case of Matilda Atoll, described by Captain 

 Beechey, is quoted as an example of this latter kind, this atoll 

 having been converted in 34 years from being a reef of rocks into 

 a lagoon island, 14 miles in length, with one of its sides covered 

 nearly all the way with high trees. Some experiments are also 

 mentioned in which it has been attempted to measure the rate of 

 increase of different kinds of corals, and as one of the results of 

 these is an instance of a growth two feet thick of coral accumu- 

 lated on the copper bottom of a vessel in the course of twenty 

 months. 



Careful examination by soundings, together with such other 

 observations as can be made on the subject, all unite in proving 



