196 
PHYSICAL HISTORY OF THE 
a very considerable thickness in the present 
direction of the valley. How far they ex¬ 
tended in width has not been ascertained by 
direct observation, for we have not seen how 
they sink away to the northward, and towards 
the south the denudation has been so complete 
that, except in the very low range of hills in 
the neighborhood of Santarem, they do not 
rise above the plain. But the fact that this 
formation once had a thickness of more than 
eight hundred feet within the limits where we 
have had an opportunity of observing it, leaves 
no doubt that it must have extended to the 
edge of the basin, filling it to the same height 
throughout its whole extent. The thickness 
of the deposits gives a measure for the colos¬ 
sal scale of the denudations by which this 
immense accumulation was reduced to its pres¬ 
ent level. Here, then, is a system of high hills, 
having the prominence of mountains in the 
landscape, produced by causes to whose agency 
inequalities on the earth’s surface of this mag¬ 
nitude have never yet been ascribed. We may 
fairly call them denudation mountains. 
At this stage of the inquiry we have to ac¬ 
count for two remarkable phenomena. First, 
