422 A JOURNEY IN BRAZIL. 
* 
founded with the mud deposits of the river. These latter, 
however, never rise so high as the ochraceous clay, but 
are everywhere confined within the limits of high and 
low water. The islands also, in the main course of the 
Amazons, consist invariably of river-mud ; while those 
arising from the intersection and cutting off of portions 
of the land by diverging branches of the main stream 
always consist of the well-known sandstones, capped by 
the ochre-colored clay. 
It may truly be said that there does not exist on the 
surface of the earth a formation known to geologists re- 
sembling that of the Amazons. Its extent is stupendous ; 
it stretches from the Atlantic shore, through the whole 
width of Brazil, into Peru, to the very foot of the Andes. 
Humboldt speaks of it " in the vast plains of the Amazons, 
in the eastern boundary of Jaen de Bracamoros," and 
says, " This prodigious extension of red sandstone in the 
low grounds stretching along the east of the Andes is 
one of the most striking phenomena I observed during 
my examination of rocks in the equinoctial regions." * 
When the great natural philosopher wrote these lines, he 
had no idea how much these deposits extended beyond 
the field of his observations. Indeed, they are not limited 
to the main bed of the Amazons ; they have been fol- 
* Bohn's edition of Humboldt's Personal Narrative, Chap. II. p. 134. Hum- 
boldt alludes to these formations repeatedly : it is true that he refers them to 
the ancient conglomerates of the Devonian age, but his description agrees so 
perfectly with what I have observed along the banks of the Amazons and 
the Rio Negro that there can be no donbt he speaks of the same thing. He 
wrote at a time when many of the results of modern geology were nn- 
known, and his explanation of the phenomena was then perfectly natural. 
The passage from which the few lines in the text are taken shows that these 
deposits extend even to the Llanos. 
