VALLEY OF THE AMAZONS. 
179 
I shall presently speak more in detail, such 
a clay bed divides the lower from the upper 
sandstone. The thickness of these sandstones 
is extremely variable. In the basin of the 
Amazons proper, they hardly rise anywhere 
above the level of high water during the rainy 
season, while at low water, in the summer 
months, they may be seen everywhere along 
the river-banks. It will be seen, however, that 
the limit between high and low water gives no 
true measure of the original thickness of the 
whole series. 
In the neighborhood of Almeirim, at a short 
distance from the northern bank of the river, 
and nearly parallel with its course, there rises 
a line of low hills, interrupted here and there, 
but extending in evident connection from Al¬ 
meirim through the region of Monte Alegre to 
the heights of Obydos. These hills have at¬ 
tracted the attention of travellers, not only 
from their height, which, because they rise ab¬ 
ruptly from an extensive plain, appears greater 
than it is, but also on account of their curious 
form, many of them being perfectly level on 
top, like smooth tables, and very abruptly 
divided from each other by low, intervening 
