338 RESUME AND THEORETICAL DISCUSSION. 
events which have happened at the very last that we can ever fully under- 
stand, and these only under specially favorable conditions. The beginning 
of all erosion is, of course, the beginning of change of relative level as 
between different parts of the earth’s crust. A uniformly and perfectly level 
surface would never suffer any change from any of the agencies which we 
class under the term “ erosive’’; but let one area be raised above another, 
and, unless this has taken place at a considerable depth beneath the surface 
of the water, the work of the rain or of the running stream, or of the waves, 
if the region affected be near the ocean’s edge, will be at once begun. The 
steeper the grade, that is, the greater the relative difference between the 
higher and the lower region, the more will the eroding power of the water 
be increased. This, then, will be the first condition to be considered, and the 
power of running water to act is, as is well known, increased in an enormously 
rapid ratio with its increase of velocity, while velocity depends primarily on 
increase of grade. 
But here another equally important factor comes in, — quantity of erosive 
material, that is, of the water; this is important with regard to subaerial 
erosion. In the case of the ocean and its work, the quantity of water remains 
the same at all points, and the erosive effect which it is able to produce de- 
pends on the character and force of its movements,— whether tidal or other, 
—and of the nature of the shore-line against which it acts; but in the case 
of subaerial erosion, or the work of rain and rivers, the quantity of the rain 
while falls is an element of prime importance. That under certain meteoro- 
logical conditions, that is, with absence of rain and wind and with a uniform 
temperature, there would be no erosion, even in a region having a surface 
of the most diversified character, is quite evident. And without rain, even 
with great changes of temperature, erosion could hardly be said to take 
place ; for although the higher portions would eventually crumble to pieces, 
the fragments could not be carried away, but would be piled up as they fell, 
gradually forming a protective cover to what was underneath. Such changes 
we may with reason suppose to be taking place on the surface of the moon ; 
and that this is so rough at the present time is proof that it is not long since 
volcanic activity ceased there, if, indeed, it has really come to an end. 
The next, and by far the most obscure and difficult part of the problem 
before us is the character of the surface as left by orographic forces, at the 
time when these ceased to act, and the region was delivered over, so to 
speak, to be worked into details by erosive agencies. We have abundant 
