=Professor Agassiz’s Investigations on Coral Islands.=--Having steamed and observed for twenty-five hundred miles among the Paumotu Islands, Prof. Alexander Agassiz says, in a second letter from the Albatross Expedition, published in the American Journal of Science, that he has seen nothing tending to show that there has anywhere been a subsidence, but that the condition of the islands does not seem to him capable of explanation on any theory except that they have been formed in an area of elevation. All the islands examined are composed of a tertiary coralliferous limestone, which has been elevated to a greater or less extent above the level of the sea, and then planed down by atmospheric agencies and submarine erosion, and the appearance of this old rock is very different from that of the modern reef rock. In these islands the rims of the great atolls, after having been denuded to the level of the sea, are built up again from the material of their two faces, so that a kind of conglomerate, or breccia, or pudding stone, or beach rock is found on all the reef flats. On the lagoon side sand bars grow into small islands and gradually become covered with vegetation. Whenever the material supplied from both sides is very abundant the land ring becomes more or less solid; the islets become islands, separated by narrow or wider cuts, until they at length form the large islands, which seem at first to be a continuous land around the rim of the lagoon, while they are often really much dissected. In time water ceases to pass through the channels, and only the marks of them are left. Few if any of the lagoons appear to be shut off from the sea as Dana and other writers have supposed. They simply have not boat passages. Unlike other coral regions, the Paumotu reefs seem to bear only a scanty life. * * * * *