A GEOLOGICAL SKETCH 143 
to what the American geologists term a pene-plain, 
from which now rise the denuded and weathered 
domes of granite, cubes or ridges of sedimentary 
rocks, and those strange rock-pillars which here and 
there rise above the forest towards the sources of 
the Rio Negro. 
The very interesting work of Professor Hartt 
and his colleagues appears to have been con- 
centrated on the north side of the great river, 
while the less extensive hills of Santarem receive 
the most meagre notice. Mr. Derby, towards the 
end of ^his careful paper, says : " The Tertiary beds 
of the southern side of the valley are, in the 
Santarem region, considerably lower than those 
of the north. The highlands behind Santarem are 
400 feet high. ... In a bed of blue clay exposed 
on the slope of these highlands, I found worm- 
tubes, the only fossils that the Tertiary beds of 
this region have yet afforded." He then goes on 
to say that the coarse sandstone beds of the plains 
about Para and in Marajo ''are certainly more 
modern and belong to the later Tertiary or the 
Quaternary." It is quite certain, therefore, that he 
could never have visited the remarkable rounded 
hills, almost buried in scoria-like masses, described 
by Spruce and cursorily examined by myself, these 
being not "behind" but 3 or 4 miles to the south- 
east of Santarem ; while those farther inland, as 
described by Spruce, are exactly like the table- 
topped Tertiary hills of Monte Alegre. If Spruce's 
observation at Caripi (near Para) of "trap rock 
penetrating into clefts of the sandstone which it 
had actually fused " be correct, it would seem to 
indicate that the Para grit is, as Spruce supposed. 
