the summits of low hills (Fig. 21), is convex upward.

In Figure 22, representing a cubic block of stone whose faces are
a yard square, how many square feet of surface are exposed to the
weather by a cubic foot at a corner a; by one situated in the
middle of an edge b; by one in the center of a side c? How much
faster will a and b weather than c, and what will be the effect on
the shape of the block?

THE COOPERATION OF VARIOUS AGENCIES IN ROCK SCULPTURE. For the
sake of clearness it is necessary to describe the work of each
geological agent separately. We must not forget, however, that in
Nature no agent works independently and alone; that every result
is the outcome of a long chain of causes. Thus, in order that the
mountain peak may be carved by the agents of disintegration, the
waste must be rapidly removed,--a work done by many agents,
including some which we are yet to study; and in order that the
waste may be removed as fast as formed, the region must first have
been raised well above the level of the sea, so that the agents of
transportation could do their work effectively. The sculpture of
the rocks is accomplished only by the cooperation of many forces.

The constant removal of waste from the surface by creep and wash
and carriage by streams is of the highest importance, because it
allows the destruction of the land by means of weathering to go on
as long as any land remains above sea level. If waste were not
removed, it would grow to be so thick as to protect the rock
beneath from further weathering, and the processes of destruction
which we have studied would be brought to an end. The very
presence of the mantle of waste over the land proves that on the