198 
PHYSICAL HISTORY OF THE 
rials are also more completely comminuted, 
and, as I have said above, contain hardly any 
large masses, though quartz pebbles are some¬ 
times scattered throughout the deposit, and oc¬ 
casionally 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 even intersects the mass of the 
clay, giving it in such instances an unques¬ 
tionably stratified character. There can be no 
question that this more recent formation rests 
unconformably upon the sandstone beds be¬ 
neath 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 accu¬ 
mulated against it; at the season of the en- 
chente , or high water, it is the only formation 
left exposed above the water level. Its thick¬ 
ness is not great; it varies from twenty or 
thirty to fifty feet, and may occasionally rise 
nearly to a hundred feet in height, though this 
is rarely the case. It is evident that this for¬ 
mation also was once continuous, stretching 
