PREVALENCE OF THE PHENOMENON.
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tion b y filtration through a thick mass o f grasses. I suggest these ideas only in the form of a d o u b t . T h e colouring principle seems t o b e in little a b u n d a n c e ; f o r I observed that the waters o f the G u a i n a o r Rio N e g r o , when subjected t o ebullition, do n o t b e c o m e b r o w n like o t h e r fluids charged with carburets o f h y d r o g e n . I t is also very remarkable, that this p h e n o m e n o n o f black waters, which might b o supposed t o b e l o n g only t o the low regions o f the torrid zone, is found also, t h o u g h rarely, o n the table-lands o f the A n d e s . T h e t o w n o f C u e n c a in the k i n g d o m o f Q u i t o , is surrounded b y three small rivers, the M a c h a n g a r a , the Rio del Matadero, and the Y a n u n c a i ; o f which the t w o former are white, and the waters o f the last are black (aguas n e g r a s ) . T h e s e waters, like those o f t h e A t a b a p o , are o f a coffee-colour b y reflection, and pale y e l l o w by transmission. T h e y are very clear, and the inhabitants o f C u e n c a , w h o drink t h e m in preference t o any other, attrib u t e their c o l o u r t o the sarsaparilla, which it is said g r o w s abundantly on the banks o f the Rio Yanunรงai. W e left the m o u t h o f the Z a m a at five in the m o r n i n g o f the 23rd o f A p r i l . T h e river c o n t i n u e d t o b e skirted o n both sides b y a thick forest. T h e mountains o n t h e east seemed gradually t o retire farther b a c k . W e passed first the m o u t h o f the R i o M a t a v e n i , and afterward an islet o f a very singular form ; a square granitic rock that rises in the middle o f the water. I t is called b y t h e missionaries E l Castillito, o r the L i t t l e Castle. Black bands seem t o indicate, that the highest swellings o f the O r i n o c o d o n o t rise at this place above eight f e e t ; and that the great swellings observed lower d o w n are o w i n g t o the tributary streams which flow i n t o it north o f the raudales o f A t u r e s and M a y pures. W e passed the night o n the right bank opposite t h e mouth o f the R i o Siucurivapu, near a r o c k called A r i c a g u a . D u r i n g the night an innumerable quantity o f bats issued from the clefts o f the r o c k , and hovered around o u r h a m mocks. O n the 24th a violent rain obliged us early t o return to o u r boat. W e departed at t w o o ' c l o c k , after having lost s o m e books, which we c o u l d n o t find in the darkness o f the n i g h t , on the rock o f Aricagua. T h e river runs straight from south to north ; its banks are l o w , and shaded o n b o t h sides