A chalky soil is one derived from limestone rocks which, when burned, yield the lime of commerce. Lime differs from chalk in not containing carbonic acid gas; this was driven off in the burning. When lime is burned it is known as quicklime; and when water is poured on this it is readily absorbed, expansion takes place, and great heat is generated.

The result is then known as hydrate of lime. When quicklime is exposed for a time to the air, it gradually absorbs carbonic acid gas, and thus reverts to a chalky or carbonate of lime condition.

Chalk or limestone (calcium carbonate) is known to geologists as organic rock, because it is made up of the remains of shells and bones of sea and freshwater fish. This may be seen by rubbing down some fragments in water and examining the dried sediment under the microscope. Minute shells, pieces of coral and sponges, and broken fragments of shells will be observed, as well as the remains of other marine creatures. Limestone hills and rocks are to be found in many parts of the world thousands of feet above sea level, and bear silent testimony to the upheavals that must have taken place on the surface of the globe in past ages. In the same way our coal seams represent ancient forests and fertile vegetation that have become submerged, and afterwards covered with deposits of other layers of soil.