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 bo con- 



