SPECIFIC TYPES OE CORAL REEFS. 73 



growing corals. The coral species that contribute the most extensive supply of calcareous material 

 to this reef-rock-manufacture are, beyond question, the more fragile and relatively quick-growing 

 species of Madrepora, and other branching varieties, upon which the storm waves strike with 

 most destructive force. 



We iiave now reached the point in this chapter which is the most eligible for an 

 enumeration of the more distinct descriptions of coral-reefs that exist, together with an exami- 

 nation of the somewhat divergent theories that have been adduced concerning their respective 

 origins. By far the most lucid and intelligible description of the various classes of these 

 structures, combined with a logical exposition of their mode of origin, is undoubtedly that 

 contained in the classical treatise on " The Structure and Distribution of Coral-Reeis," by 

 Mr. Charles Darwin, published in the year 1842, which work may be said to have laid the 

 foundation of the imperishable reputation of that distinguished naturalist. Within the last 

 decade some of the more important of Darwin's theories of coral-reef formation have been 

 vigorously attacked, so much so that at the present day there are divided biological camps 

 regarding the tenability or otherwise of the Darwinian interpretations. Such being the case, 

 it has been deemed advisable by the author to enter at some considerable length into an ex- 

 amination of the conflicting evidence adduced, and to quote largely from the writings of 

 the authorities at variance. 



The specific varieties of coral-reefs that receive universal recognition at the hands of 

 biologists are, as originally classified by Mr. Darwin, referred to three distinct categories. 

 These are denominated respectively as — i, "Lagoon-Islands" or "Atolls"; 2, "Barrier" or 

 " Encircling- Reefs " ; 3, "Fringing" or "Shore-Reefs." The Lagoon-Islands or Atolls, in Mr. 

 Darwin's own words, are "vast rings of coral-rock, often many leagues in diameter, here and 

 there surmounted by a low verdant island with dazzling white shores, bathed on the outside 

 by the foaming breakers of the ocean, and on the inside surrounding a calm expanse of water, 

 which, from reflection, is generally of a bright but pale-green colour." Occasionally the 

 entire ring of coral-rock is surmounted by an uninterrupted circle of upraised vegetated, in- 

 habitable land ; more frequently, the coral-rock ring includes a number of irregularly detached 

 islets ; while in innumerable instances the rock-circle with the enclosed lagoon is entirely 

 submerged at high tide, and only indicates its existence by the presence of broken water 

 around its periphery, and by the characteristic emerald-green tint of the water that overlies 

 its superficies. The contour of these atolls may vary in form from an almost perfect circle, 

 or oval, to every conceivable pattern of irregularity, and, in dimensions, from a lew hundred 

 yards to a diameter of over eighty miles. Milla-dou-Madon, in the Maldiva Archipelago, of 

 an irregular elongate outline, is cited in Mr. Darwin's work as being no less than eighty- 

 eight miles long by a little less than twenty broad. In these atolls, where the enclosed lagoon 

 is more or less completely surrounded by a ring of elevated land, there is usually a channel 



