PHYSICAL HISTORY OF THE AMAZONS. 405 
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 Ama- 
zonian 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 met in Para on our re- 
turn from our respective explorations, and 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 for- 
mations 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 compan- 
ions, proceeded up the coast to Para. I was surprised to 
find at every step of my progress the same geological phe- 
nomena which had met me at Rio. 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 re- 
cent a period. And here let me say that the facts I now 
state are by no means exclusively the result of my own 
investigations. They are in great part due to Major 
