202 
PHYSICAL HISTORY OF 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 I^a, the Jutahy, 
the Jurua, the Japura, and the Purus. On the 
banks of the Japura, 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 Humboldt not only describes 
them from a higher point on this same river, 
but also from the valley of the Orinoco. Fi¬ 
nally, 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 northern 
affluents of the Amazons. The observations 
of Martius, those of Gardner, and the recent 
survey above alluded to, made by my assistant, 
Mr. St. John, of the valley of the Rio Gu- 
ruguea and that of the Rio Paranahyba, show 
that the great basin of Piauhy is also identi¬ 
cal in its geological structure with the lateral 
valleys of the Amazons. The same is true 
of the large island of Marajo, lying at the 
mouth of the Amazons. And yet I believe that 
