PHYSICAL HISTORY OF THE AMAZONS. 423 
lowed along the banks of its tributaries to the south and 
north as far as these have been ascended. They occur on 
the margins of the Huallaga and the Ucayale, on those 
of the lea, the Hyutahy, the Hyurua, the Hyapura, and 
the Purus. On the banks of the Hyapura, where Major 
Coutinho has traced them, they are found as far as the 
Cataract of Cupati. I have followed them along the Rio 
Negro to its junction with the Rio Branco ; and Hum- 
boldt not only describes them from a higher point on this 
same river, but also from the valley of the Orinoco. Finally, 
they may be tracked along the banks of the Madeira, the 
Tapajos, the Xingu, and the Tocantins, as well as on the 
shores of the Guatuma, the Trombetas, and other north- 
ern affluents of the Amazons. The observations of Mar- 
tins, those of Gardner, and the recent survey above alluded 
to, made by my assistant, Mr. St. John, of the valley of 
the Rio Guruguea and that of the Rio Paranahyba, show 
that the great basin of Piauhy is also identical in its 
geological structure with the lateral valleys of the Ama- 
zons. The same is true of the large island of Marajo, 
lying at the mouth of the Amazons. And yet I believe 
that even this does not cover the w^hole ground, and 
that some future writer may say of my estimate, as I 
have said of Humboldt's, that it falls short of the truth ; 
for, if my generalizations are correct, the same formation 
will be found extending over the whole basin of the Para- 
guay and the Rio de la Plata, and along their tributaries, 
to the very heart of the Andes. 
Such are the facts. The question now arises, How 
were these vast deposits formed ? The easiest answer, 
and the one which most readily suggests itself, is that of 
a submersion of the continent at successive periods, to 
