. Ecological animal geography; an authorized, rewritten edition based on Tiergeographie auf ockologischer grundlage. Zoogeography -- Geographical distribution; Animal ecology. 212 MARINE ANIMALS meters above sea level, and often more or less interrupted by pas- sages (Fig. 41). The largest atoll, Sunadiva, is 42 by 32 miles with a perimeter of 121 miles and contains over a hundred subsidiary islets and a lagoon up to 100 m. deep which connects with the open sea by 40 channels. Darwin63 explained these varied reef formations with a comprehen- sive theory based on the following considerations: (
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. Ecological animal geography; an authorized, rewritten edition based on Tiergeographie auf ockologischer grundlage. Zoogeography -- Geographical distribution; Animal ecology. 212 MARINE ANIMALS meters above sea level, and often more or less interrupted by pas- sages (Fig. 41). The largest atoll, Sunadiva, is 42 by 32 miles with a perimeter of 121 miles and contains over a hundred subsidiary islets and a lagoon up to 100 m. deep which connects with the open sea by 40 channels. Darwin63 explained these varied reef formations with a comprehen- sive theory based on the following considerations: (1) that reef corals • ' •—1 ^^SillaisssSsSwa^^ SES^ V =^E^i^=S^^iSs^^^S:;^ Fig. 41.—The atoll of Pinaki, Tuamotu Islands. After Agassiz. flourish only to a depth of 40-60 m.; (2) that the growth is most rapid on the outer face of the reef; and (3) that variations in the level of the sea bottom take place. From the first consideration, reefs cannot rise to the surface from the oceanic depths, but must be confined to submarine banks, such as the tops of submerged volcanos, or on the coast of existing land. On stationary and on rising coasts, the reefs are consequently of the fringing type (Fig. 42, I). But when mainland coast or islands are sinking, the outer, more rapidly growing edge of. Fig. 42.—Origin of various forms of coral reefs according to Darwin: I, fringing reef; II, barrier reef; III, atoll. Cross sections above, outline below, in each case. 1, 2, sea level in I and II respectively. the reef will be separated more and more from the shore, and a channel or lagoon without corals is consequently produced between the reef and the original coast, i.e., a barrier reef is formed (Fig. 42, II). When the submergence is long-continued, the island itself may disappear, and only the reef, now an atoll, remains (Fig. 42, III). "Atolls are the grave stones of sunken islands." All intergradations between barrier reefs surrounding islands and atolls may be found