MR. DARWIN'S REEF-FORMATION THEORIES. 75 



respect to fringing, or shore-reefs, there is little in their structure which needs explanation ; and 

 their name expresses their comparatively small extension. They differ from barrier reefs in 

 not lying far from the shore, and in not having within them a broad channel of deep water. 

 Reefs, allied in most respects to fringing-reefs, also occur around submerged banks of sediment 

 and of worn-down rock ; and others are scattered quite irregularly where the sea is very 

 shallow." 



With the assistance of the foregoing authoritative and generally accepted statements, the reader, 

 voyaging in coral seas, should experience no difficulty in assigning such reefs as he may explore to 

 their proper categories. Progress may now be made towards an enumeration and examination of 

 the somewhat varying constructive interpretations that have been associated with their origin. The 

 fringing-reef,/>£r s<?, presents no constructive difficulties ; its origin, as an in-shore or shallow-water 

 coral-bank, which has sprung up wheresoever the depth and associated conditions are favourable 

 to its development, being clearly apparent. It may, nevertheless, as presently shown, represent 

 the initial growth of the two other more complex reef modifications. Passing on to the consideration 

 of the lagoon-islands, or atolls, — " those singular rings of coral-land which rise abruptl}' out of the 

 unfathomable ocean," as Mr. Darwin elsewhere describes them — we enter into the arena of fierce 

 debate. Prior to Darwin's explanation, the theory of atoll formation most generally received 

 was that atolls were based on submarine craters. The untenability of this theory was, however, 

 clearly demonstrated b}' Mr. Darwin, who pointed out that the abnormal size and shapes of many 

 of the most typical atolls precluded such an interpretation. Bow atoll, which is five times as long 

 as it is broad ; Menchicoff Island, with three loops, together, sixty miles ; Rimsky Korsakoff, 

 narrow, crooked, and fifty-four miles long ; and, finally, Milla-dou-Matte, in the Maldiva atoll 

 group, eighty-eight miles in length and from ten to twenty broad, are cited as being entirely 

 incompatible with a crater origin. A still more valid objection to the acceptance of the ''crater" 

 interpretation is associated by Mr. Darwin with the fact that, in order to establish its truth, all of 

 the innumerable atoll-bearing centres would have to lie at approximately the same level beneath 

 the sea. If such structures actually represented the basis of all atolls, some of these craters 

 would undoubtedly be upraised above the sea-level. " Nevertheless," Darwin remarks, " if the 

 rim of a crater afforded a basis at the proper depth, I am far from denying that a reef like a 

 perfectly characterised atoll might not be formed on it. Some such, perhaps, now exist; but it 

 is incredible that the greater number could have thus originated." 



An earlier theory, first put forward by Chamisso, concerning the origin of atolls, is next 



discussed. That writer suggests that, as the more massive kinds of corals prefer the surf, 



the outer portions of a reef will first reach the surface, and consequently form a ring. This 



theory is dealt with by Mr. Darwin, thus — "I have previously remarked that a reef growing 



on a detached bank would tend to assume an atoll-like structure ; if, therefore, corals were 



to grow up from a bank some fathoms submerged in a deep sea, having steep sides and a 



L 2 



