Agassiz's most important contribution to science after his settlement in America was his report upon the Florida Reefs, a strip of rocks fringing the southern coast of Florida, which had long puzzled the American naturalists, who had so far been unable to agree as to their geological formation. The Coast Survey of the United States was particularly anxious to have the question of their formation settled, both from a practical point of view in regard to navigation, and for scientific reasons, and Agassiz was asked to make an exploration of that region in the interests of the Government.

Agassiz accepted the commission with great eagerness, and made an exhaustive study of the reefs, arriving at the conclusion that these fringes of rocks, which were separated by deep channels, were not a freak of nature, but that the whole peninsula of Florida had been formed by successive circles of these coral reefs, the everglades being only filled up channels, and that the soft soil, now so shifting and uncertain, would in time present the firm appearance indicated by the older portions.

The report was valuable to the Coast Survey, as it determined the nature of the soil, and indicated what localities might be available as offering stable foundations for light-houses, signal stations, and the like.

The life of Agassiz from the time of his coming to America was one of ceaseless activity, and his lectures at Cambridge and in Charleston, S. C., where he resided for some time, formed only a small part of his work. His contributions to science were of the greatest value, and during the first fifteen years of his residence in the United States his essays on the geographical and geological distributions of animals; on the natural history of the United States; on the glacial phenomena of Maine, and kindred subjects, served to advance in a marked degree the sciences of geology and zoölogy, which were still in a process of formation, while the of several scientific schools, which have since attained to eminence, were likewise attributable to the same master-mind.

In 1865 Agassiz made a journey to Brazil, having the twofold object of obtaining a needed rest from his usual work, and of making collections for the Museum of Natural History; and this expedition was fruitful in scientific interest.

He remained in Brazil something over a year, and was able to make a most satisfactory collection of Brazilian fishes, bringing away with him two thousand specimens obtained from the Amazon and its tributary streams and lakes. It was also a great source of pleasure to Agassiz to find, in the Brazilian tropics, evidences of the great ice-period, which proved to him that the glaciers had once covered that region, where now the rays of the sun are so powerful as to endanger life.

In 1871 Agassiz started out on another expedition, having for its object the study of the animals of the sea, for which purpose it was proposed to dredge the coast-waters down the Atlantic and up the Pacific as far as San Francisco. This work was especially attractive to Agassiz, as he believed that the study of the deep-sea animals would reveal many of the missing links between the fossil world and living species; and, beside this, he also expected to find evidences that the glacial phenomena, familiar to the northern hemisphere, were also to be found in the southern, and thus add a stronger proof that his glacial theory was correct.

The active work of the expedition began as soon as the Gulf Stream was reached, with the study of the Sargassum, or fields of drifting seaweed, which abound in those regions, and which was filled with minute forms of life. Agassiz was able to make a very satisfactory study of the Sargassum, and this good beginning was followed up by dredgings in the Barbadoes, which revealed some living sponges, so much like the fossils he had previously studied that Agassiz felt that his theories of deep-sea dredging were already bringing him a golden harvest.

On the coast of Montevideo Agassiz found strong evidences of glacial action, and on the coast of the Argentine Republic many fossils were obtained from the huge boulders which were scattered everywhere; while the geological formation of the coast of the Straits of Magellan gave still further evidence of the truth of his favorite theory in regard to glaciers. Here he found a moraine composed of boulders, pebbles, and gravel, polished and grooved, and bearing all the signs of glacial action, while the ice- and snow-fields glittering upon the slopes of the mountains could only remind him of the glaciers of the Alps, thus proving to Agassiz that the ice period had extended over the southern as well as the northern continents, coming in both cases from the poles and retreating eventually in the same directions. Agassiz studied the glacier regions of the south for several weeks, and then the vessel proceeded on her way to new fields of investigation.

A trip was made to the Galapagos Islands, interesting to naturalists because of their recent origin and their peculiar varieties of plants and animals, some of which are different from any known in other parts of the world; and here Agassiz made some important studies on the formation of volcanic islands, of which this group formed an instance.

The voyage was then continued up the Pacific, the original plan to proceed to San Francisco being carried out by their reaching that city in August, 1872, having accomplished very nearly all that they set out to do. This was Agassiz's last scientific excursion; and, after his return to Cambridge, he busied himself with plans for a School of Natural History to be established somewhere on the coast of Massachusetts, and which was to be in operation in the summer, for the benefit of pupils and teachers from all over the country; an important plan, as it has resulted in the founding of various summer schools which have greatly advanced the study of science.

Lectures, essays, and study filled up another year, and in December, 1873, the work of the great student came to a close, and he passed away from earth leaving behind him the fruits of a well-spent life in which selfish aims and enjoyments had no share, and which was of inestimable value to science.