V 
A GEOLOGICAL SKETCH 
137 
little vegetation beyond a few scattered grasses on 
the summit, and a few minute ferns and Selaginellas 
under the shade of the projecting cope. A similar 
hill, with a much broader flat top, apparently of the 
same elevation, rises on the other side of the valley 
of the Irura, to eastward, or N.E. ^ E. of the first. 
From the summits of these hills there was a good 
though distant view of the Serras of Monte Alegre, 
among which might be distinguished many table- 
topped summits, some of them apparently much 
higher than those of Santarem. On referring to 
Mr. Wallace's account of his visit to Monte Alegre, 
I find the following description of one of these 
hills : We now saw the whole side of the mountain, 
along its summit, split vertically into numerous rude 
columns, in all of which the action of the atmosphere 
was more or less discernible. They diminished and 
increased in thickness as the soft and hard beds 
alternated, and in some places appeared like globes 
standing on pedestals, or the heads and bodies of 
giants." And of a cave which he went to explore : 
"The entrance is a rude archway, 1 5 or 20 feet 
high ; but what is most curious is a thin piece of 
rock which runs completely across the opening, 
about 5 feet from the ground, like an irregular flat 
board. This stone has not fallen into its present 
position, but is a portion of the solid rock harder 
than the rest, so that it has resisted the force which 
cleared away the material above and below it." 
There is precisely the same kind of white sand- 
stone, with alternating hard and soft strata, some 
hundreds of miles higher up the Amazon, in a low 
table-land extending parallel to the left bank of the 
Rio Negro, from its mouth to I know not how far 
