1888.] on Structure, Origin, and Distribution of Coral Beefs, &c. 259 



from the top of a submerged mounta'n have an area of one square mile, 

 then on reaching the surface of the waves there will be a shallow 

 depression in the centre owing to the more rapid growth of the outer 

 edge. Such an atoll will have, if it be a square, four miles of outer 

 reef for the supply of coral sand and other debris, and these being 

 washed and blown into the one square mile of shallow lagoon it is 

 likely to become filled up, the result being a small island with dry 

 lagoon in which may be found deposits of sulphate of lime, magnesian 

 and phosphatic rocks, and guano — all these testifying to the great 

 age of the island and absence of subsidence in the region. It is only 

 atolls with a diameter of less than two miles that thus become filled up. 

 In other and larger plantations, rising from a more extensive bank, 

 the conditions are very different. In this larger atoll — say four 

 miles square — there is now only one mile of outer reef to each square 

 mile of lagoon, instead of four miles of outer reef to the one square mile 

 of lagoon in the smaller atoll. Only one-fourth of the detrital matter 

 and food enters the larger lagoon, from the outside, per square mile 

 of lagoon, and hence there is proportionally less living coral ; the 

 solvent agencies predominate and the lagoon is widened and deepened. 

 Growing seawards on the outer face and dissolving away in the 

 lagoon, the whole expands after the manner of a fairy ring, and the 

 ribbon of reef or land can never in consequence increase beyond a 

 half or three-quarters of a mile in width, it being usually much 

 less. ' 



Atolls may occur far away from any other land, but it more 

 frequently happens that they are arranged in linear groups, in this 

 respect resembling volcanic islands. Extensive banks may be crowded 

 with small atolls, like the Northern Maldives ; or a bank may be 

 occupied by one great and perfect atoll 20 to 40 miles in diameter, 

 like some of the Southern Maldives and the Paumotus. In some 

 instances the large atolls aj^pear to have resulted from the growth 

 and coalescence of the smaller marginal atolls ; esi)ecially does this 

 seem to have been the case with the large Southern Maldives. 



The outer slopes vary greatly in different reefs, and in different 

 parts of the same reef. When there is deep water beyond, the reef 

 very often extends out with a gentle slope to a depth of 25 to 40 

 fathoms, and is studded with living coral, the bosses and knobs 

 becoming larger in the deeper water farthest from the reef, where 

 there are great overhanging cliffs, which eventually fall away by 

 their own weight and form a talus on which the reef may proceed 

 further outwards. Occasionally there is a very steep descent almost 

 at once from the outer edge. Thus the deeper the water beyond, the 

 more slowly will the reef extend seawards. In reefs with a very 

 gentle slope outside, the corals are frequently overhanging at depths 

 of six or seven fathoms, for in these instances the lower part of the 

 sea-face of the reef is rendered unsuitable for vigorous growth, in 

 consequence of the sand which is carried in by waves coming over 

 the comparatively shallow depths outside. 



