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 whole rocks weather more rapidly than their waste is removed.  The destruction of the land is going on as fast as the waste can be carried away.

We have now learned to see in the mantle of waste the record of the destructive action of the agencies of weathering on the rocks of the land surface.  Similar records we shall find buried deeply among the rocks of the crust in old soils and in rocks pitted and decayed, telling of old land surfaces long wasted by the weather.  Ever since the dry land appeared these agencies have been as now quietly and unceasingly at work upon it, and have ever been the chief means of the destruction of its rocks.  The vast bulk of the stratified rocks of the earth’s crust is made up almost wholly of the waste thus worn from ancient lands.

In studying the various geological agencies we must remember the almost inconceivable times in which they work.  The slowest process when multiplied by the immense time in which it is carried on produces great results.  The geologist looks upon the land forms of the earth’s surface as monuments which record the slow action of weathering and other agents during the ages of the past.  The mountain peak, the rounded hill, the wide plain which lies where hills and mountains once stood, tell clearly of the