But when that energy became feeble, and when at last it ceased, the once powerful structure descended again to that lower level of the inorganic, and subject to all its laws. Then, what the ocean could not do by the violence of its waves, it was all-potent to do by the corroding and dissolving power of its calmer lagoons. Ever eating, corroding, and dissolving, the back waters of the original fringing reef—the mere pools and channels left by the outrageous sea as it dashed upon the shore—were ceaselessly at work, aided by the high temperature of exposure to blazing suns, and by the gases evolved from decaying organisms. Thus the enlarging area of these pools and channels spread out into wide lagoons, and into still wider protected seas. They needed no theory of subsidence to account for their origin or for their growth. They would present the same appearance in a slowly-rising, a stationary, or a slowly-sinking area. Their outside boundary was ever marching farther outward on submarine shoals and banks, and ever as it advanced in that direction its rear ranks were melted and dissolved away. Their inner boundary—the shores of some island or of some continent—might be steady and unmoved, or it might be even rather rising instead of sinking. Still, unless this rising were such as to overtake the advancing reef, the lagoon would grow, and, if the shores were steady, it would widen as fast as the face of the coral barrier could advance. Perhaps, even if such a wonderful process had ever occurred to Darwin—even if he had grasped this extraordinary example of the "give and take" of Nature—of the balance of opposing forces and agencies which is of the very essence of its system, he would have been startled by the vast magnitude of the operations which such an explanation demanded. In its incipient stages this process is not only easily conceivable, but it may be seen in a thousand places and in a thousand stages of advancement. There are islands without number in which the fringing reef is still attached to the shore, but in which it is being "pitted," holed, and worn into numberless pools on the inner surfaces where the coral is in large patches dead or dying, and where its less soluble ingredients are being deposited in the form of coral sand. There are thousands of other cases where the lagoon interval between the front of the reef and the shores has been so far widened that it is taking the form of a barrier, as distinguished from a fringing reef, and where the lagoon can be navigated by small boats. But when we come to the larger atolls, and the great seas included between a barrier-reef and its related shores, the mind may well be staggered by the enormous quantity of matter which it is suggested has been dissolved, removed, and washed away. The breadth of the sheltered seas between barrier-reefs and the shore is measured in some cases not by yards or hundreds of yards, not by miles, but by tens of miles, and this breadth is carried on in linear directions, not for hundreds of miles, but for thousands. And yet there is one familiar idea in geol-
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A GREAT LESSON.