The boat channel indicated between F and G remains to be accounted for. As the reef flat widens it results in a great area covered by shallow water at high tide level and partly bare at low. Exposed to a tropic sun life is impossible for any but a few specialised forms, the rock is unprotected from such wave motion as there is, and from boring organisms, which agencies quickly reduce its level. Strong currents also flow over the surface, for the breakers throw water over the raised edge which may have to travel several miles before reaching a gap through which it can return to the sea, and this with tidal currents cause swift rivers of muddy water to flow over the reef flat parallel with the shore. The obvious result is the hollowing out of a boat channel[50], and the accumulation in it of great quantities of mud and sand, which in many places form the greater part of its actual surface, but which eventually are swept out to sea. The presence of certain marine flowering plants of grass-like form (_Cymodocea_ and other genera) assists, if not wholly responsible for, the formation of such accumulations by binding the mass together by their strong and tangled roots and rhizomes. When the channel has become broad and deep enough, coral growth may resume sway in it, sometimes to such an extent as almost to block it up again. I need offer no proof of the formation of a reef flat and precipice by coral growth, the thing is obvious, at least in the case of ordinary fringing reefs. But the hollowing out of the boat channel, and with it that of other lagoons enclosed by coral, is less obvious, and it is natural to assign to a feature so distinctive of coral reefs an origin more directly dependent on the laws of coral growth. The proof comes from a consideration of the simplest case. Do we know of any reefs where solution and abrasion have formed these characteristic features without aid from coral growth? We do. _Plate XXXIII_ [Illustration: Fig. 72. A portion of a reef flat shewing _sections_ of contained shells] [Illustration: Fig. 73. The under surface of the same stone shewing that the reef is composed of a loosely cohering mass of shells and broken coral. In the centre of the mass is a shell easily recognisable as one living in abundance nowadays, _Strombus fasciatus_]