18 



panied us to the Rio Negro ; speaking of it as 

 of an habitual evil, which it was easy to bear. 

 The fits were violent, but of short duration. 

 He was sometimes seized with them when lying 

 along in the boat under a shelter of branches of 

 trees, sometimes when exposed to the burning 

 rays of the sun on an open beach. These tertian 

 agues are attended with great debility of the 

 muscular system ; yet we find poor ecclesiastics 

 on the Oroonoko, who support for several years 

 these calenturitas, or tercianas : their effects are 

 not so fatal as those, which are experienced from 

 fevers of much shorter duration in temperate 

 climates. 



I have just mentioned the noxious influence 

 on the salubrity of the atmosphere, which is at- 

 tributed by the natives, and even the missiona- 

 ries, to the bare rocks. This opinion is the more 

 worthy of attention, on account of it's being con- 

 nected with a physical phenomenon lately ob- 

 served in different parts of the globe, which has 

 not yet been sufficiently explained. Among the 

 cataracts, and wherever the Oroonoko, between 

 the missions of Carichana and of Santa Barbara, 

 periodically washes the granitic rocks, they 

 become smooth, black, and as if coated with 

 black-lead. The colouring matter does not pene- 

 trate the stone, which is coarse-grained granite, 

 containing a few solitary crystals of hornblende. 

 Considering the primitive formation of Atures 



