The survey of the Macclesfield bank proved that a considerable number of reef corals are to be found alive at depths {390}ranging from 30 to 50 fathoms.[408] To give one example:—In the dredging No. 50, depth 32 to 35 fathoms, living examples of the following genera of corals were obtained: _Madrepora_, _Montipora_, _Psammocora_, _Pavonia_, and _Astraeopora_. CORAL REEFS AND ATOLLS.—In many regions of the tropical seas, banks and islands are found which are built up of blocks of coral, coral detritus, and altered or modified limestone. These are the famous coral reefs of which so much has been said and written during the last half-century. There can be little doubt that the superficial strata of these formations are entirely due to the action of coral-forming animals and plants living in warm, shallow sea-water. [Illustration: FIG. 172.—Plan of Minikoi Atoll in Laccadive Archipelago. _A_, the land elevated above the level of high-water mark; _Ch_, the boat channel; _5 fm_, the five fathom line; _2 fm_, the two fathom line; _L_, the lagoon with a maximum depth of 7 fathoms; _R_, the reef continuing the circle on the east side of the atoll, awash at high tides. (After Stanley Gardiner.)] Three classes of coral reefs are usually recognised: the "fringing reefs" which follow the contour of the coast at a distance of a few hundred yards, and are separated from the beach at low tide by sand flats or a shallow lagoon; the "barrier reefs," following the contour of the coast less regularly than the fringing reefs, but at a much greater distance, and separated from the beach by a lagoon of sufficient depth to serve as a harbour for ships of great size; and, finally, the "atolls," which are ring-shaped, or broken circlets of low islands enclosing a lagoon which is, in some cases, of considerable depth. It was observed by the early surveyors that in many cases the sea-bottom slopes downwards steeply or almost precipitously from the outer edge of the barrier reefs and atolls to very great {391}depths—to depths, in fact, at which reef-forming corals do not live. It seems obvious, therefore, that the atolls and barrier-reefs are resting upon some stratum which could not possibly have been formed by reef-building organisms at the same relative position it has now, and the questions arose, What is the substratum and how was it formed?