Derby.] 1°^ [Feb. 21, 



region about Alcnguer, ascended the river Curua de Alenguer as far as 

 the Bern fica fall, and afterwards revisited the lower Tapajos. Dr. Freitas 

 and myself ascended the Trombetas and I afterwards revisited Mara jo. 

 The Devonian fossils have been studied by Mr. Richard Rathbun, while I 

 have myself determined those of the Silurian and Carboniferous. 



Having been intimately associated with Prof. Hartt in all the Amazonian 

 work, I can claim but little originality in the conclusions drawn from the 

 observations and presented in this article, the most of them having been 

 presented by my illustrious teacher in his various publications, or brought 

 out in our discussions on the subject, in such a way that it is now impos- 

 sible to determine the authorship of each idea. The work of the last ex- 

 plorations by Messrs. Smith, Freitas and myself, in which Prof. Hartt, had 

 no part, was mainly the determination of the character and age of the 

 Erere uplift, and of the extension and relations of the various Palaeozoic 

 deposits on the northern side of the Amazonas. It is proper to state that 

 in regard to the Cretaceous age of the Erere sandstone and the date of the 

 elevation of the anticlinal, Prof. Hartt reserved his opinion for a more 

 careful examination of the evidence that I had to present on that point, 

 than he was ever able to make. I am confident, however, that if he had 

 made such an examination, I should have been able to convince him of the 

 accuracy of my observations and conclusions. 



The river known to geographers by the name Amazonas has, like many 

 other rivers, various names which are applied b) r the inhabitants along its 

 banks to different parts of its course. These popular designations of 

 Amazonas, or Baixo (Lower) Amazonas, Solimoes and Maranon, mark ap- 

 proximately three sections of the valley, which are very distinct in physical 

 characteristics and have ver} r different geological histories. They may, 

 therefore, be advantageously retained to designate the lower, middle and 

 upper portion of the great river. 



The differences in these three sections are due to the relations of the 

 valley with the component parts of the South American continent ; so that 

 in order to understand the structure of the valley, we must bear in mind 

 the general features, long since recognized, of that continent. Tins is 

 composed of three distinct mountainous regions, more or less united by 

 elevated plains, in which are excavated the great depressions occupied by 

 the fluvial systems of the Orinoco, Amazonas and Rio de la Plata. The 

 Andes form a long, narrow strip of great elevation, along the western 

 coast, and the mountains of Brazil and of Guiana, considerably less ele- 

 vated than the Andes, occupy extensive areas in the eastern and northern 

 portions of the continent. The space between these three elevated regions 

 or nuclei of the continent is occupied by vast elevated plains, generally 

 less than three thousand feet high, except in a narrow strip between the 

 highlands of Brazil and Guiana, in which the continuity of the plains is 

 entirely interrupted by the depressed valley of the Amazonas. It is also 

 to be noted that between the Andes and the two elevated regions of the 

 eastern part of the continent, the continuity of the plateaux is almost de- 



