43 



meets the advanced buttresses of the Cordillera 

 of Parima. This obstacle is the cause of cata- 

 racts far more considerable, and more injurious 

 to the navigation, than all the Pongos of the 

 Upper Maragnon, since, as we have shown 

 above, they are proportionally nearer to the 

 mouth of the river. I have entered into these 

 geographical details, to prove by the example 

 of the two greatest rivers of * the New World, 

 1st, that it cannot be ascertained in an absolute 

 manner, that, beyond a certain number of 

 toises, a certain height above the level of the 

 sea, rivers are not navigable ; 2dly, that the 

 rapids are not always occasioned, as several 

 treatises of general topography affirm, by the 

 height of the first obstacles, by the first lines of 

 ridges, which the waters have to surmount near 

 their sources. 



The northernmost of the great cataracts of the 

 Oroonoko is the only one bounded on each side 

 by lofty mountains. The left bank of the river 

 is generally lower, but makes part of a plane, 

 which rises again west of Atures, toward the 

 Peak of Uniana, a pyramid nearly three thou- 

 sand feet high, and placed on a wall of rock 

 with steep slopes. The situation of this solitary 

 peak in the plain contributes to render it's aspect 

 more imposing and majestic. Near the mission, 



* We may add the instances of the Ohio and Dnieper. 



