Mary Ellis, Age 11, Of Memos., Mich., for her question:

How many crusts has the earth?

The solid earth, we are told, is made of layers of minerals one outside another like the skins of an onion. The outer crust reaches down scme 30 to 50 miles, and it is made of the lightest of the earth's minerals. Below the outer crust is a shell of denser minerals called the mantle. This layer is about 600 miles thick.

The mantle of dense rocks rests on a layer of still heavier rocks, which is: perhaps 1200 miles thick. At the very center of the globe is the earth's core. It begins about 2000 miles beneath our feet, and it seems to be made mostly from heavy nickel and iron.