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 I^a, 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 whole 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 suiruvsts itself, is that of 

 a submersion uf the continent at successive periods, to 



