To-morrow we leave Manaos in the “Ibicuhy,” in order to ascend the Rio Negro as far as Pedreira, where the first granitic formation is said to occur. _December 27th._—On board the “Ibicuhy.” There was little incident to mark our day yesterday, and yet it was one full of enjoyment. The day itself was such as rarely occurs in these regions; indeed, I should say it is the only time, during the whole six months we have passed on the Amazons, when we have had cool weather with a clear sky. Cool weather here is usually the result of rain. As soon as the sun shows his face the heat is great. But yesterday a strong wind was blowing down the Rio Negro; and its usually black, still waters were freshened to blue, and their surface broken by white caps. It is a curious fact in the history of this river, that, while tributary to the Amazons, it also receives branches from it. A little above its junction with the Solimoens, the latter sends several small affluents into the Rio Negro, the entrance to which we passed yesterday. The contrast between their milky-white waters and the clear, dark, amber tint of the main river makes them very conspicuous. It would seem that this is not a solitary instance of river formation in this gigantic fresh-water system; for Humboldt says, speaking of the double communication between the Cassiquiare and the Rio Negro, and the great number of branches by which the Rio Branco and the Rio Hyapura enter into the Rio Negro and the Amazons: “At the confluence of the Hyapura there is a much more extraordinary phenomenon. Before this river joins the Amazons, the latter, which is the principal recipient, sends off three branches, called Uaranapu, Manhama, and Avateparana, to the Hyapura, which is but a tributary stream. The Portuguese astronomer, Ribeiro, has proved this important fact. The Amazons gives waters to the Hyapura itself before it receives that tributary stream.” So does it also to the Rio Negro.