Jill Francis, age 11, of Selkirk, Manitoba, Canada, for her question:

Is it true that some rocks are made of shells?

All the lime type rocks G ere formed from seashells, including, the limestones, chalks and waxy marbles. In some of these minerals, it is possible to trace the shapes and forms of the shells. But as a rule, they are invisible, even through a good magnifying glass. This is because most of the fossilized shells were made by microscopic marine creatures in the remote past. Time and pressure have mashed them out of shape.

The sea has teemed with various microscopic shell builders since life on earth began. Then, as now, they extracted limy calcium type chemicals from the water to build their microscopic shells. As countless generations died, their shells piled thick layers on the ocean floors. Where seabeds lifted high and dry above the water, these sedimentary layers became deposits of chalk and limestone. In mountainous regions, some of these were metamorphosized    completely remodeled by heat and pressure to form slabs of waxy marble.