Geology of the Valley. 559 



ley was foreshadowed. The superficial Cretaceous strata 

 up the Paraiia-pura, at the Pongo de Manseriche, and from 

 Tomependa up the remarkable longitudinal valley of the 

 Upper Maranon to Balsas, into the department of An- 

 cachs, would indicate that so much at least of the Great 

 River began to exist in the Early Tertiary. Without 

 doubt, during the Cretaceous period, the Atlantic and Pa- 

 cific were continuous oceans, flowing over not only the 

 Panama isthmus, but also over all Equatorial America, 

 save a few islands and reefs. We are not surprised, there- 

 fore, to find the same Cretaceous (and even Miocene) spe- 

 cies on both sides of the Andes.* 



The vast basin (whether Carboniferous or Cretaceous I 

 will not say) formed by the rise of the Andes and the raet- 

 amorphic regions on the north and south received an im- 

 mense sheet of colored clays, sands, and sandstones. This 

 deposit, unique in its extent and origin, is known as the 

 Amazonian Tertiary formation. It was the sediment of 

 a brackish Mediterranean, or of a quiet lake to which 

 brackish water had occasional access. The argillaceous and 

 loamy beds are universal ; the sandstone has been reduced 

 by subsequent denudation, and is now nearly confined to 

 the Lower Amazons.f Excepting this sandstone, the ma- 

 terial is so thoroughly comminuted that a pebble is a rar- 

 ity. The Maraiion Indians, upon returning from up the 

 Ucayali and other tributaries, bring home rocks to sharpen 

 their knives. I have seen, however, concretions, nodular 

 and stalactiform, strikingly similar to the marly concre- 

 tions noticed by Darwin in the Pampean mud. 



Previous to the expedition of the writer across the con- 



* Mr. Bland informs me, after an examination of my land-shells, that the 

 general aspect of the living Bulimi from the Peruvian Andes is remarkably 

 like the Lower Californian. 



t Vesicular, ferruginous sandstone occurs far up the Madeira and Negro. 

 I am not aware of its existence in any part of the Maraiion region. 



