A GEOLOGICAL SKETCH 139 
The bed of the Amazon has plainly been 
excavated, by whatever means, in this white sand- 
stone and in the subjacent Para grit/ At abrupt 
points or capes, on the Amazon, the grit crops out 
to view, as at Paricatuba, Serpa, Poraquecoara, 
etc.; and even where the shore is low and alluvial, 
and there are subsidiary channels at the back of the 
main stream, on penetrating far enough inland one 
is sure to come on the ancient rocky margin of 
the Amazon. Thus at Manaquiry, lying S. of 
the Amazon about fifty miles above the Rio 
Negro, where there is a perfect labyrinth of lakes 
and channels, we find at the back of all a wall of 
nearly horizontally stratified Para grit rising 30 
feet above the present high-water mark, and often 
so hidden by vegetation as to seem from the water 
a steeply-sloping bank ; but I traced it for many 
miles continuously, and found it to be here and 
there indented with deep bays or cirques ; and I 
cannot doubt that anciently the main Amazon laved 
this rocky wall, and in time of flood rose to a level 
with its top. There is said to be a similar wall a 
little farther westward at Menacapurii on the 
northern side of the Amazon. A few volcanic 
blocks, mostly isolated, repose on the wall at 
Manaquiry. 
At Caripi I thought I saw indications of the 
sandstone rock on which the scoriae reposed having 
been affected by heat at the point of contact, but I 
sought in vain for such indications elsewhere. 
^ [The American geologists consider the Para sandstone to be a more recent 
deposit. — Ed,] 
