DENUDING POWER OF RIVERS. 
lOi 
the same hills, and the newest of all in the channel of the ex¬ 
isting river where there is usually gravel alone, although in 
some cases a naivrow strip of solid lava shares the bottom of 
the valley with the river. 
The proportion of extinct species of quadrupeds is more 
numerous in the fossil remains of the gravel No. 1 than in 
that indicated as No 2; and in No. 3 they agree more close¬ 
ly, sometimes entirely, with those of the existing fauna. 
The usual absence or rarity of organic remains in beds of 
loose gravel and sand is partly owing to the friction which 
originally ground down the rocks into small fragments, and 
partly to the porous nature of alluvium, which allows the 
free percolation through it of rain-water, and promotes the 
decomposition and removal of fossil rojnains. 
The loose transported matter on the surface of a large part 
of the land now existing in the temperate and arctic regions 
of the northern hemisphere, must be regarded as being in a 
somewhat exceptional state, in consequence of the important 
part which ice has played in comparatively modern geologic¬ 
al times. This subject will be more specially alluded to 
when we describe, in the eleventh chapter, the deposits called 
“ glacial.” 
Denuding Power of Eivers affected by Rise or Pall of Land.— 
It has long been a matter of common observation that most 
rivers are now cutting their channels through alluvial depos¬ 
its of greater depth and extent than could ever have been 
formed by the present streams. From this fact it has been 
inferred that rivers in general have grown smaller, or be¬ 
come less liable to be flooded than formerly. It may be true 
that in the history of almost every country the rivers have 
been both larger and smaller than they are at the present 
moment. For the rainfall in particular regions varies ac¬ 
cording to climate and physical geography, and is especially 
governed by the elevation of the land above the sea, or its 
distance from it and other conditions equally fluctuating in 
the course of time. But the phenomenon alluded to may 
sometimes be accounted for by oscillations in the level of the 
land, experienced since the existing valleys originated, even 
where no marked diminution in the quantity of rain and in 
the size of the rivers has occurred. 
We know that many large areas of land are rising and 
others sinking, and unless it could be assumed that both the 
upward and downward movements are everywhere uniform, 
many of the existing hydrographical basins ought to have 
the appearance of having been temporary lakes first filled 
with fluviatile strata *and then partially re-excavated. 
