_April 7th._—To-day the lecture was upon the physical features of South America, something with reference to the geological and geographical work in which Mr. Agassiz hopes to have efficient aid from his younger assistants. So much of the lecture consisted of explanations given upon geological maps that it is difficult to record it. Its principal object, however, was to show in what direction they should work in order to give greater precision to the general information already secured respecting the formation of the continent. “The basin of the Amazons, for instance, is a level plain. The whole of it is covered with loose materials. We must watch carefully the character of these loose materials, and try to track them to their origin. As there are very characteristic rocks in various parts of this plain, we shall have a clew to the nature of at least some portion of these materials. My own previous studies have given me a special interest in certain questions connected with these facts. What power has ground up these loose materials? Are they the result of disintegration of the rock by ordinary atmospheric agents, or are they caused by the action of water, or by that of glaciers? Was there ever a time when large masses of ice descended far lower than the present snow line of the Andes, and, moving over the low lands, ground these materials to powder? We know that such an agency has been at work on the northern half of this hemisphere. We have now to look for its traces on the southern half, where no such investigations have ever been made within its warm latitudes; though to Darwin science is already indebted for much valuable information concerning the glacial phenomena of the temperate and colder portions of the South American continent. We should examine the loose materials in every river we ascend, and see what relation they bear to the dry land above. The color of the water in connection with the nature of the banks will tell us something. The waters of the Rio Branco, for instance, are said to be milky white; those of the Rio Negro, black. In the latter case the color is probably owing to the decomposition of vegetation. I would advise each one of our parties to pass a large amount of water from any river or stream along which they travel through a filter, and to examine the deposit microscopically. They will thus ascertain the character of the detritus, whether from sand, or lime, or granite, or mere river mud formed by the decomposition of organic matter. Even the smaller streams and rivulets will have their peculiar character. The Brazilian table-land rises to a broad ridge running from west to east, and determining the direction of the rivers. It is usually represented as a mountain range, but is in fact nothing but a high flat ridge serving as a water-shed, and cut transversely by deep fissures in which the rivers flow. These fissures are broad in their lower parts, but little is known of their upper