136 NOTES OF A BOTANIST 
but although I have traversed the eastern base of 
the Andes, from about f S. lat. nearly to the 
Equator, along the rivers Huallaga, Pastasa, and 
Bombonasa, nowhere did I see the volcanic boulders 
of the Amazon reproduced. Even among the 
volcanoes themselves of the Andes, I have never 
seen any scoriae so perfectly vitrified on the surface 
as those of Santarem, nor any lavas so completely 
fused as those of Etna and Vesuvius. The tufas 
of Cotopaxi have, in fact, been boiled rather than 
fused. But if we go still farther to the west, 
beyond the Andes and the American coast, we 
come on a group of volcanic islands (the Galapagos) 
lying upon the Equator, where glazed scoriae like 
those of the Amazon abound. 
Before pursuing the considerations to which 
these facts lead, let us return to Santarem and 
see how far my scanty observations will aid us in 
ascertaining what has happened to the stratified 
rocks, and how the bed of the Amazon may have 
been excavated therein. Due south from the Serra 
d' Irura, and with three lowish intervening ridges, 
there is a curious isolated table-topped hill, the 
abrupt and naked southern side of which looks at 
a distance like the ruins of a Gothic castle, from its 
being cleft into masses resembling towers. On 
examination it is found to consist of white sand- 
stone, in horizontal layers ; and the thinnish top 
layer being much harder than the next subjacent 
layers, it has resisted atmospheric or other decom- 
posing agencies to a greater degree, and projects 
all round like the coping of a wall. The edges of 
similar thin compact layers form projecting rims 
here and there on the vertical surface. There was 
