I have said nothing of Mr. Agassiz’s observations on the character of the soil since we left Rio, thinking it best to give them as a whole. Along the entire length of the coast he has followed the drift, examining it carefully at every station. At Bahia it contained fewer large boulders than in Rio, but was full of small pebbles, and rested upon undecomposed stratified rock. At Maceió, the capital of the province of Alagôas, it was the same, but resting upon decomposed rock, as at Tijuca. Below this was a bed of stratified clay, containing small pebbles. In Pernambuco, on our drive to the great aqueduct, we followed it for the whole way; the same red clayey homogeneous paste, resting there on decomposed rock. The line of contact at Monteiro, the aqueduct station, was very clearly marked, however, by an intervening bed of pebbles. At Parahyba do Norte the same sheet of drift, but containing more and larger pebbles, rests above a decomposed sandstone somewhat resembling the decomposed rock of Pernambuco. In the undecomposed rock below, Mr. Agassiz found some fossil shells. In the neighborhood of Cape St. Roque we came upon sand-dunes resembling those of Cape Cod, and wherever we sailed near enough to the shore to see the banks distinctly, as was frequently the case, the bed of drift below the shifting superficial sands above was distinctly noticeable. The difference in color between the white sand and the reddish soil beneath made it easy to perceive their relations. At Ceará, where we landed, Mr. Agassiz had an opportunity of satisfying himself of this by closer examination. At Maranham the drift is everywhere conspicuous, and at Pará equally so. This sheet of drift which he has thus followed from Rio de Janeiro to the mouth of the Amazons is everywhere of the same geological constitution. It is always a homogeneous clayey paste of a reddish color, containing quartz pebbles; and, whatever be the character of the rock in place, whether granite, sandstone, gneiss, or lime, the character of the drift never changes or partakes of that of the rocks with which it is in contact. This certainly proves that, whatever be its origin, it cannot be referred to the localities where it is now found, but must have been brought from a distance. Whoever shall track it back to the place where this peculiar red soil with its constituent elements forms the primitive rock, will have solved the problem. I introduce here a letter written by Mr. Agassiz, a few days later, to the Emperor, which will better give his views on the subject. A BORD DE L’ICAMIABA, SUR L’AMAZONE, le 20 Aout, 1865.