420 A JOURNEY IN BRAZIL. 
denudation more extensive than any thus far recorded 
in the annals of geology, which has given rise to all the 
most prominent hills and mountain-chains along the north- 
ern bank of the river. Before seeking an explanation of 
these facts, let us look at the third and uppermost deposit. 
This deposit is essentially the same as the Rio drift ; 
but in the north it presents itself under a somewhat dif- 
ferent aspect. As in Rio, it is a clayey deposit, containing 
more or less sand, and reddish in color, though varying 
from deep ochre to a brownish tint. It is not so abso- 
lutely destitute of stratification here as in its more south- 
ern range, though the traces of stratification are rare, 
and, when they do occur, are faint and indistinct. The 
materials are also more completely comminuted, and, as I 
said above, contain hardly any large masses, though quartz 
pebbles are sometimes scattered throughout the deposit, 
and occasionally a thin seam of pebbles, exactly as in the 
Rio drift, is seen resting between it and the underlying 
sandstone. In some places this bed of pebbles intersects 
even the mass of the clay, giving it, in such instances, 
an unquestionably stratified character. There can be no 
doubt that this more recent formation rests unconform- 
ably upon the sandstone beds beneath it; for it fills all 
the inequalities of their denudated surfaces, whether they 
be more or less limited furrows, or wide, undulating de- 
pressions. It may be seen everywhere along the banks 
of the river, above the stratified sandstone, sometimes 
with the river-mud accumulated against it ; at the season 
of the encliente, or high water, it is the only formation left 
exposed above the water-level. Its thickness is not great ; 
it varies from twenty or thirty to fifty feet, and may occa- 
sionally rise nearly to a hundred feet in height, though this 
