A GEOLOGICAL SKETCH 141 
position in the geological series is perfectly well 
established. 
We now come to the series of flat-topped hills 
extending for about 150 miles on the north bank of 
the Lower Amazon to beyond Monte Alegre, and 
a few of precisely similar form and structure on the 
south side, as described above by Spruce. These 
consist of horizontal beds of sandstones and clays, 
and are often about 1000 feet high. They are 
isolated and have suffered a large amount of 
circumdenudation ; but as no fossils have been 
found in them, their exact age is unknown, though 
they undoubtedly belong to the Tertiary forma- 
tion. 
In the lower land behind, and sometimes between, 
these there is exposed a large extent of coarse 
massive sandstone, with intercalated beds of shale. 
These rise into rounded hills farther inland, and 
also near the river in the Serras of Erere ; and 
they are all more or less inclined and disturbed, 
besides being traversed in various directions by 
trap dykes, which in some parts are very numerous, 
while in others the volcanic rock seems to occur in 
intrusive layers. In some places these dykes stand 
above the surface like ruined walls ; in others they 
have been denuded more than the adjacent rock so 
as to form sunken channels. These sandstones 
contain fossilised wood and abundance of dicotyle- 
donous leaves fairly well preserved. Hence it is 
concluded that they cannot be older than the 
Cretaceous age ; while their being always more or 
less disturbed and penetrated by trap dykes shows 
that they are much older than the overlying softer 
sandstones, which are always horizontal and never 
