nearly three-quarters-is screened by water, chiefly oceans and glaciers.

The earth's crustal surface-man's domain and vulnerable to a variety of natural and man-made occurrences-is tremendously versatile. It produces our food. It provides our basic fuels and metals. It supports our steel and concrete structures and our vehicles.

The thinnest part of this crust, the soil, is a fairly recent addition. The most productive, it is the most fragile. It must sustain life, but man reduces its vitality and hastens erosive processes.

Each day, rain and running water sweep 8 million tons of the land's substance into the sea. The material carried seaward contains natural nutrients. It also contains natural and man-made contaminants.

In a sense, our land must be considered a nonrenewable resource because some areas are being depleted far faster than nature is creating new fertile areas of productivity.

The natural riches stored in the earth's crust are finite, yet they are being depleted at everincreasing rates. Some experts say that the world is overpopulated; others contend that it easily can support more life. Both agree, however, that better conservation practices are absolutely necessary on a sustained global basis if environmental and resource crises are to be avoided.

Man has altered numerous features of the earth's surface for his own purposes. But these changes have not always been to his advantage.

Despite man's efforts to modify some of the earth's features, ours remains a dynamic planet.