PBEVALENCE OF THE PHENOMENON. 825 



tion by fJtration through a thk I mass of grasses. I suggest 

 these ideas only in the form of a doubt. The colouring 

 principle seems to be in little abundance ; for I observed 

 that the waters of the Ghiaina or Rio Negro, when subjected 

 to ebullition, do not become brown like other fluids charged 

 with carburets of hydrogen. 



It is also very remarkable, that this phenomenon of black 

 waters, which might be supposed to belong ouly to the low 

 regions of the torrid zone, is found also, though rarely, on 

 the table-lands of the Andes. The town of Cuenca in the 

 kingdom of Quito, is surrounded by three small rivers, the 

 Machangara, the Rio del Matadero, and the Yanuncai ; of 

 which the two former are white, and the waters of the last 

 are black (aguas negras). These waters, like those of the 

 Atabapo, are of a coffee-colour by reflection, and pale yellow 

 by transmission. They are very clear, and the inhabitants of 

 Cuenca, who drink them in preference to any other, attri- 

 bute their colour to the sarsaparilla, which it is said grows 

 abundantly on the banks of the Rio Tanun9ai. 



We left the mouth of the Zama at five in the morning of 

 the 23rd of April. The river continued to be skirted on 

 both sides by a thick forest. The mountains on the east 

 seemed gradually to retire farther back. We passed first 

 the mouth of the Rio Mataveni, and afterward an islet of a 

 very singular form ; a square granitic rock that rises in the 

 middle of the water. It is called by the missionaries El 

 Castillito, or the Little Castle. Black bands seem to indi- 

 cate, that the highest swellings of the Orinoco do not rise 

 at this place above eight feet ; and that the great swellings 

 observed lower down are owing to the tributary streams 

 which flow into it north of the raudales of Atures and May- 

 pures. We passed the night on the right bank opposite the 

 mouth of the Rio Siucurivapu, near a rock called Aricagun. 

 During the night an innumerable quantity of bats issued 

 from the clefts of the rock, and hovered around our hum- 

 mocks. 



On the 24th a violent rain obliged us early to return to our 

 boat. We departed at two o'clock, after having lost some 

 books, which we could not find in the darkness of the night, 

 on the rock of Aricagua. The river runs straight from 

 to north ; its banks are low, and shaded on both sides 



