PHYSICAL HISTORY OF THE AMAZONS. 421 
is rarely the case. It is evident that this formation also was 
once continuous, stretching over the whole basin at one 
level. Though it is now worn down in many places, and 
has wholly disappeared in others, its connection may be 
readily traced ; since it is everywhere visible, not only 
on opposite banks of the Amazons, but also on those 
of all its tributaries, as far as their shores have been ex- 
amined. I have said that it rests always above the sand- 
stone beds. This is true, with one exception. Wherever 
the sandstone deposits retain their original thickness, as 
in the hills of Monte Alegre and Almeyrim, the red clay 
is not found on their summits, but occurs only in their 
ravines and hollows, or resting against their sides. This 
shows that it is not only posterior to the sandstone, but 
was accumulated in a shallower basin, and consequently 
never reached so high a level. The boulders of Erere 
do not rest on the stratified sandstone of the Serra, but 
are sunk in the unstratified mass of the clay. This 
should be remembered, as it will presently be seen that 
their position associates them with a later period than 
that of the mountain itself. The unconformability of 
the ochraceous clay and the underlying sandstones might 
lead to the idea that the two formations belong to distinct 
geological periods, and are not due to the same agency 
acting at successive times. One feature, however, shows 
their close connection. The ochraceous clay exhibits a 
remarkable identity of configuration with the underlying 
sandstones. An extensive survey of the two, in their 
mutual relations, shows clearly that they were both de- 
posited by the same water-system within the same basin, 
but at different levels. Here and there the clay forma- 
tion has so pale and grayish a tint that it may be cn>- 
