elevated; with the aid of the dredge I satisfied myself of their identity. With these facts before me I cannot doubt that the oesars of the United States consist essentially of glacial material remodeled by the sea; while farther inland, though here and there reaching the sea-coast, we have unchanged glacial drift deposit. At some points the alteration is so slight as to denote only a momentary rise of the sea. Under these circumstances one would naturally look for fossils in the drift, and M. Desor, in company with M. de Pourtales, was the first to find them, at Brooklyn, in Long Island, which lies to the south of New York. They were imbedded in a glacial clay deposit, having all the ordinary character of such deposits, with only slight traces of stratified sand. It is true that the greater number of these fossils (all belonging to species now living on the coast) were broken into angular fragments, not excepting even the thick tests of the Venus mercenaria. . . The suburb of Boston where I am living (East Boston) is built on an island, one kilometer and a half long, extending from north to southeast, and varying in width at different points from two to six or seven hundred metres. Its height above the sea-level is about sixty feet. This little island is composed entirely of glacial muddy deposit, containing scratched pebbles mixed with larger boulders or blocks, and covered also with a considerable number of boulders of divers forms and dimensions. At East Boston you cannot see what underlies this deposit; but no doubt it rests upon a rounded mass of granite, polished and grooved like several others in Boston harbor. . . In our journey to Niagara, Mr. Desor and I assured ourselves that the river deposits, in which, among other things, the mastodon is found with the fresh-water shells of Goat Island, are posterior to the drift. It is a fact worth consideration that the mastodons found in Europe are buried in true tertiary formations, while the great mastodon of the United States is certainly posterior to the drift. . .In another letter I will tell you something of my observations upon the geographical distribution of marine animals at different depths and on different bottoms, and also upon the relations between this distribution and that of the fossils in the tertiary deposits. . .* (* I have left out a portion of this letter which appeared in the first edition of the book, because I learned that the facts there given concerning the deposit of Zostera marina were not substantiated, and that Agassiz consequently did not forward the letter in its first form. The remainder of this chapter appears in this edition for the first time.--E.C.A.)