Now, every reef of the fringing class is breached by a narrow gateway in front of the smallest rivulet, even if dry during the greater part of the year, for the mud, sand, or gravel, occasionally washed down kills the corals on which it is deposited. We can easily see how an island fronted only on one side, or on one side with one end or both ends encircled by barrierreefs, might after longcontinued subsidence be converted either into a single walllike reef, or into an atoll with a great straight spur projecting from it, or into two or three atolls tied together by straight reefs all of which exceptional cases actually occur. As the reefbuilding corals require food, are preyed upon by other animals, are killed by sediment, cannot adhere to a loose bottom, and may be easily carried down to a depth whence they cannot spring up again, we need feel no surprise at the reefs both of atolls and barriers becoming in parts imperfect. I cannot refrain from once again remarking on the singularity of these complex structures a great sandy and generally concave disk rises abruptly from the unfathomable ocean, with its central expanse studded, and its edge symmetrically bordered with oval basins of coralrock just lipping the surface of the sea, sometimes clothed with vegetation, and each containing a lake of clear water! And as by our theory the areas including atolls and barrierreefs are subsiding, we ought occasionally to find reefs both dead and submerged. It is remarkable that in all these cases, the dead reefs and portions of reef lie at nearly the same depth, namely, from six to eight fathoms beneath the surface, as if they had been carried down by one uniform movement. Thus have we traced the history of these great rings of coralrock, from their first origin through their normal changes, and through the occasional accidents of their existence, to their death and final obliteration. Now in this map we see that the reefs tinted pale and darkblue, which have been produced by the same order of movement, as a general rule manifestly stand near each other. I have marked with vermilion spots all the many known active volcanos within the limits of this same map. Their entire absence from every one of the great subsiding areas, coloured either pale or dark blue, is most striking and not less so is the coincidence of the chief volcanic chains with the parts coloured red, which we are led to conclude have either long remained stationary, or more generally have been recently upraised. Although a few of the vermilion spots occur within no great distance of single circles tinted blue, yet not one single active volcano is situated within several hundred miles of an archipelago, or even small group of atolls. Hence in these cases it would appear, that volcanos burst forth into action and become extinguished on the same spots, accordingly as elevatory or subsiding movements prevail there. Taking a final view of the map, and bearing in mind the statements made with respect to the upraised organic remains, we must feel astonished at the vastness of the areas, which have suffered changes in level either downwards or upwards, within a period not geologically remote. It would appear also, that the elevatory and subsiding movements follow nearly the same laws. Throughout the spaces interspersed with atolls, where not a single peak of high land has been left above the level of the sea, the sinking must have been immense in amount. The sinking, moreover, whether continuous, or recurrent with intervals sufficiently long for the corals again to bring up their living edifices to the surface, must necessarily have been extremely slow. We may thus, like unto a geologist who had lived his ten thousand years and kept a record of the passing changes, gain some insight into the great system by which the surface of this globe has been broken up, and land and water interchanged. The one block of greenstone, moreover, on the northern island must be excepted. From this point of view the aspect of the island equalled the expectations raised by the many wellknown descriptions of its beautiful scenery.
