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APPENDIX II. 
of tlie results of a visit to Keeling Atoll 1 (known also as the 
Cocos Island). As the letter is only a preliminary note, it 
is difficult to analyse or appreciate the writer’s arguments, 
so it may suffice to say that he is convinced that ‘ several 
important characters of these islands escaped the attention 
of Mr. Darwin,’ that ‘ these features throw considerable 
light on the mode of origin of lagoon islands, and give 
no support to the theory of subsidence.’ According to 
Mr. Guppy’s description ‘ Keeling Atoll consists essentially 
of a ring of horseshoe or crescentic islands, including a 
lagoon, and presenting their convexities seawards. The 
crescentic form is possessed in varying degrees by different 
islands ; some of the smaller ones are perfect horseshoe 
atollons and inclose a shallow lagoonlet, others again ex- 
hibit only a semi-crescentic form, while the larger islands 
have been produced by the union of several islands of this 
shape.’ He states that from the effects of gales, &c., the 
islands are constantly altering in shape, and expresses his 
decided opinion that the ‘ small atolls and horseshoe 
islands only assume their horseshoe form after the island 
has been thrown up by the waves.’ This is due to the 
sand and ddbris, which are swept along by a current, 
accumulating under the lee of the ends of a shoal on the 
face of which the current impinges, so that the island 
tends to extend, both laterally and to leeward, and thus 
gradually to assume, more or less, the shape of a crescent 
or horseshoe. Some estimates are given of the amount of 
material transported by the currents. 
Outside the seaward edge of the present reef, Mr. Guppy 
has observed a series of submerged lines of growing corals 
separated from each other by sandy intervals. Thus the 
outward extension of a reef is effected ‘ not so much 
by the seaward growth of the present edge of the reef, as 
by the formation outside of it of a line of growing corals 
1 See ch. i. sect. i. of the present work. 
