red, unstratified clay, with pebbles and occasional boulders, overlying the rock in place. Mr. Orestes St. John, who, taking the road through the interior, has visited, with the same objects in view, the valleys of the Rio San Francisco and the Rio das Velhas, and also the valley of Piauhy, gives the same account, with the exception that he found no erratic boulders in these more northern regions. The rarity of erratic boulders, not only in the deposits of the Amazons proper, but in those of the whole region which may be considered as the Amazonian basin, is accounted for, as we shall see hereafter, by the mode of their formation. The observations of Mr. Hartt and Mr. St. John are the more valuable, because I had employed them both, on our first arrival in Rio, in making geological surveys of different sections on the Dom Pedro Railroad, so that they had a great familiarity with those formations before starting on their separate journeys. Recently, Mr. St. John and myself having met at Pará on returning from our respective journeys, I have had an opportunity of comparing on the spot his geological sections from the valley of the Piauhy with the Amazonian deposits. There can be no doubt of the absolute identity of the formations in these valleys.
Having arranged the work of my assistants, and sent several of them to collect and make geological examinations in other directions, I myself, with the rest of my companions, proceeded up the coast to Pará. I was surprised to find at every step of my progress the same geological phenomena which had met me at Rio. As the steamer stops for a number of hours, or sometimes for a day or two, at Bahia, Maceio, Pernambuco, Parahiba, Natal, Ceara, and Maranham, I had many opportunities for observation. It was my friend Major Coutinho, already an experienced Amazonian traveller, who first told me that this formation continued through the whole valley of the Amazons, and was also to be found on all of its affluents which he had visited, although he had never thought of referring it to so recent a period. And here let me interrupt the course of my remarks to say, that the facts recorded in this article are by no means exclusively the result of my own investigations. They are in great part due to this able and intelligent young Brazilian, a member of the government corps of engineers, who, by the kindness of the Emperor, was associated with me in my Amazonian expedition. I can truly say that he has been my good genius throughout the whole journey, saving me, by his previous knowledge of the ground, from the futile and misdirected expenditure of means and time often inevitable in a new country, where one is imperfectly acquainted both with the people and their language. We have worked together in this investigation; my only advantage over him being my greater familiarity with like phenomena in Europe and North America, and consequent readiness in the practical handling of the facts, and in perceiving their connection. Major Coutinho's assertion, that on the banks of the Amazons I should find the same red, unstratified clay as in Rio and along the southern coast, seemed to me at first almost incredible, impressed as I was with the generally received notions as to the ancient character of the Amazonian deposits, referred by Humboldt to the Devonian, and by Martins to the Triassic period, and considered by all travellers to be at least as old as the Tertiaries. The result, however, confirmed his report, at least so far as the component materials of the formation are concerned; but, as will be seen hereafter, the mode of their deposition, and the time at which it took place, have not been the same at the north and south; and this difference of circumstances has modified the aspect of a formation essentially the same throughout. At first sight, it would indeed appear that this formation, as it exists in the valley of the Amazons, is identical with that of Rio; but it differs from it in the rarity of its boulders, and in showing occasional
