loo NOTES OF A BOTANIST 
when the river is low, but in the time of flood it is 
probably a mere rapid. The rock seemed to me 
to be clay-slate, of a purplish-grey colour, rarely 
reddish. The strata dip to S.S.E. at about io°, 
and the sections of the principal planes of cleavage 
run E.S.E. and N.E. The uppermost strata, as 
seen in adjacent declivities, are thin, shaly, and 
arenaceous ; and they are overlaid by a soft sand- 
stone, in thick strata, whether conformally or not I 
could not ascertain. On the top of a sandstone 
hill to west of the fall are strewn a few dioritic 
blocks, quite like those seen elsewhere in the 
Amazon valley. 
A little above the first fall, granite rocks begin 
to appear on the left bank, and from thence upwards 
there is no other rock, the second and all the upper 
falls being over granite. The rocks, whether of 
slate or granite, over which the water falls are 
coated with a black varnish, in some places with a 
lurid yellow tinge. I have since seen apparently 
the same kind of deposit at the cataracts of the 
Orinoco, where it had previously been seen and 
described by Humboldt. He supposed it to be 
peculiar to rivers of white or muddy water, found- 
ing his opinion on the absence of any such deposit 
on the granite rocks in the black waters of the Rio 
Negro. But the Aripecuru has as clear water as 
the Rio Negro ; and at the cataracts of the 
Huallaga, whose waters are still whiter than those 
of the Orinoco, there are no varnished rocks. I 
suppose, therefore, that the deposit is owing to 
some mineral held in solution (not merely in 
suspension) in the white waters of the Orinoco 
and the black waters of the Aripecuru. 
