R. Rathbun— Geology 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 Rio Amazonas, popularly called the Mara- 
fion, Solimdes and Baixo Amazonas, or upper, median an 
lower courses, there exist not only topographical differences, but 
also very marked differences in ol ical bie te si Atter 
been discovered in the lower valley, the immediate subject of 
him 
_ his article. The most important ionalusians recorded by 
are the followin 
The peers ee asc composing the plateau and moun- 
tain range between Guayana and Brazil, and the central Bra- 
zilian a and thus Taciaie the Lower Amazonian basin 
on the north, and forming its higher lands on the south, may 
be lyelen into two series—a lower one, consisting of highly 
crystalline rocks, and an upper one, of generally non-crystalline 
rocks, The former, constitutes the most of the Guay- 
anian plateau, and forms the base of that of Brazil, consists of 
gneiss, gneiss-granite ad syenite, and has been referred by 
Prof. Hartt to the Laurentian. The Serra do Mar and the Serra 
do papa ee farther south in Brazil, are se up of the 
same formatio 
The sack or upper series, composed mostly of quartzites, 
metamorphic schists and crystalline Section 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 Bilurian are — as before, the itacolumites 
and — schists of Minas Ger The metamorphic rocks 
deira, between ge be gi g 
regions, thus defined, me approximately t the borders of the 
ancient channel, which existed between the geet be islands of 
Brazil, and in which were laid down, without great changes of 
level, or disturbances, the newer Eecmations from the Upper 
Silurian to the Cretaceous inclusi sive. 
There is a certain concordance in “Seow amet between the 
Am. Jour. So1.—Tutrp Serres, Vou. XVII, No. 102,—Jcyz, 1 
32 
