R. Rathhun — Oeology of the Lower Amazonas. 465 



to the surrounding table-lands and mountain chains, which direct 

 their courses. Mr. Derby endeavors to show that between the 

 three sections of the Eio Amazonas, popularly called the Mara- 

 iion, Solimoes and Baixo Amazonas, or upper, median and 

 lower courses, there exist not only topographical ditferences, but 

 also very marked diiferences in geological structure. After 

 briefly describing the general geological histor^^ of the Amazo- 

 , nian region, as brought out hy ProK Hartt, the author enters 

 into a detailed account of the several formations, that have 

 been discovered in the lower valley, the immediate subject of 

 his article. The most important conclusions recorded by him 

 are the following : 



The metamorphic deposits composing the plateau and moun- 

 tain range between Guayana and Brazil, and the central Bra- 

 zilian plateau, and thus bordering the Lower Amazonian basin 

 on the north, and forming its higher lands on the south, may 

 be divided into two series — a lower one, consisting of highly 

 crystalline rocks, and an upper one, of generally non-crystalline 

 rocks. The former, which constitutes the most of the Guay- 

 anian plateau, and forms the base of that of Brazil, consists of 

 gneiss, gneiss-granite and syenite, and has been referred by 

 Prof. Hartt to the Laurentian. The Serra do Mar and the Serra 

 do Mantiqueira, farther south in Brazil, are made up of the 

 same formation. 



The second or upper series, composed mostly of quartzites, 

 metamorphic schists and crystalline limestones, probably repre- 

 sents both the Huronian and Lower Silurian, as an apparent 

 difference in age is exhibited in the exposures of these rocks. 

 To the Lower Silurian are referred, as before, the itacolumites 

 and talcose schists of Minas Geraes. The metamorphic rocks 

 are generally well exposed in the falls and rapids of the several 

 tributaries of the Amazonas, the upper, or non-crystalline series 

 being usually the first reached in ascending these rivers. 



The southern edge of the metamorphic deposits of the Guay- 

 anian plateau, beginning near the Atlantic, in about 1^ north, 

 extends a little south of west to near the confluence of the Eios 

 Negro and Branco, between latitudes 1° and 2° south ; the 

 northern edge of the same rocks in the plateau of Brazil pre- 

 sents a line of exposures, which pass the Tocantins, between 

 8° and 4° S., the Tapajos, between 4° and 5° S., and the Ma- 

 deira, between 8° and 9° S. The edges of the metamorphic 

 regions, thus defined, mark approximately the borders of the 

 ancient channel, which existed between the primitive islands of 

 Brazil, and in which were laid down, without great changes of 

 level, or disturbances, the newer formations from the Upper 

 Silurian to the Cretaceous inclusive. 



There is a certain concordance in stratification between the 



