The Superintendent of the Coast Survey never had cause to regret the carte-blanche he had thus given. A few weeks, with the facilities so liberally afforded, gave Agassiz a clew to all the phenomena he had been commissioned to examine, and enabled him to explain the relation between the keys and the outer and inner reefs, and the mud swamps, or more open channels, dividing them, and to connect these again with the hummocks and everglades of the main-land. It remains to be seen whether his theory will hold good, that the whole or the greater part of the Florida peninsula has, like its southern portion, been built up of concentric reefs. But his explanation of the present reefs, their structure, laws of growth, relations to each other and to the main-land, as well as to the Gulf Stream and its prevailing currents, was of great practical service to the Coast Survey. It was especially valuable in determining how far the soil now building up from accumulations of mud and coral debris was likely to remain for a long time shifting and uncertain, and how far and in what localities it might be relied upon as affording a stable foundation. When, at the meeting of the American Association in the following spring, Agassiz gave an account of his late exploration, Dr. Bache, who was present, said that for the first time he understood the bearing of the whole subject, though he had so long been trying to unravel it. The following letter was written immediately after Agassiz's return. TO SIR CHARLES LYELL. CAMBRIDGE, April 26, 1851. . . .I have spent a large part of the winter in Florida, with a view of studying the coral reefs. I have found that they constitute a new class of reefs, distinct from those described by Darwin and Dana under the name of fringing reefs, barrier reefs, and atolls. I have lately read a paper upon that subject before the American Academy, which I shall send you as soon as it is printed. The case is this. There are several concentric reefs separated by deep channels; the peninsula of Florida itself is a succession of such reefs, the everglades being the filled-up channels, while the hummocks were formerly little intervening islands, like the mangrove islands in the present channels. But what is quite remarkable, all these concentric reefs are upon one level, above that of the sea, and there is no indication whatever of upheaval. You will find some observations upon upheavals, etc., in Silliman, by Tuomey; it is a great mistake, as I shall show. The Tortugas are a real atoll, but formed without the remotest indication of subsidence. Of course this does not interfere in the least with the views of Darwin, for the whole ground presents peculiar features. I wish you would tell him something about this. One of the most remarkable peculiarities of the rocks in the reefs of the Tortugas consists in their composition; they are chiefly made up of CORALLINES, limestone algae, and, to a small extent only, of real corals. . .