The Fire-made Rocks, both Plutonic and Volcanic;
The Water-and-Fire-made Rocks.
The first of these—Water-made Rocks—may be subdivided into three classes. These are,—
I. Flint Rocks; II. Clay Rocks; III. Lime Rocks.
This is not a book in which it would be wise to go closely into the mineral nature of rocks. Two or three leading thoughts may, however, be given.
Does it not seem strange that the hard and solid rocks should be in great measure formed of the same substances which form the thin invisible air floating around us?
Yet so it is. There is a certain gas called Oxygen Gas. Without that gas you could not live many minutes. Banish it from the room in which you are sitting, and in a few minutes you will die.
This gas makes up nearly one-quarter by weight of the atmosphere round the whole earth.
The same gas plays an important part in the ocean; for more than three-quarters of water is oxygen.