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On the Structure and Distribution of Coral Reefs
On the Structure and Distribution of Coral Reefs Author:Charles Darwin Purchase of this book includes free trial access to www.million-books.com where you can read more than a million books for free. This is an OCR edition with typos. Excerpt from book: CHAPTER I. ATOLLS OR LAGOON-IST.ANDS. Section I.?Keeling Atoll. Corals on the outer margin.?Zone of Nulliporze exterior reef.?Islets ? Coral-conglo... more »merate.?Lagoon calcareous sediment.?Scari and Holo- thurte subsisting on corals.?Changes in the condition of the reefs and islets.?Probable subsidence of the atoll.?Future state of the lagoon. Keeling or Cocos atoll is situated in the Indian Ocean, in 12 5' S., and longitude 90 55' E. : a reduced chart of it from the survey of Captain Fitzroy and the Officers of H.M.S. Beagle, is given in Plate I., Fig. 10. The greatest width of this atoll is nine miles and a half. Its structure is in most respects characteristic of the class to which it belongs, with the exception of the shallowness of the lagoon. The accompanying woodcut represents a vertical section, supposed to be drawn at low water from the outer coast across one of the low islets (one being taken of average dimensions) to within the lagoon. A.?Level of the sea at low water : where the letter A is placed, the depth is twenty- five fathoms, and the distance rather more than one hundred and fifty yards from the edge of the reef. B.?Outer edge of that flat part of the reef, which dries at low water : the edge either consists of a convex mound, as represented, or of rugged pomts, like those a little farther seaward, beneath the water. C.?A flat of coral-rock, covered at high water. D.?A low projecting ledge of brecciated coral-rock washed by the waves at high water. E.?A slope of loose fragments, reached by the sea only during galess the upper part, which is from six to twelve feet high, Is clothed with vegetation. The surface of the islet gently slopes to the lagoon. F.?Level of the lagoon at low water. The section is true to the scale in a horizo...« less