A somewhat curious calculation has been made, to illustrate the different modes of working of these two mighty powers--Fire and Water. The amount of land swept away each year in mud, and borne to the ocean by the River Ganges, was roughly reckoned, and also the amount of land believed to have been upheaved several feet in the great Chilian earthquake. It was found that the river, steadily working month by month, would require some four hundred years to carry to the sea the same weight of material, which in one tremendous effort was upheaved by the fiery underground forces. Yet we must not carry this distinction too far. Fire does not always work suddenly, or water slowly; witness the slow rising and sinking of land in parts of the earth, continuing through centuries; and witness also the effects of great floods and storms. The crust of the earth is made of rock. But what is rock made of? Certain leading divisions of rocks have been already considered: The Water-made Rocks; 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_. It plays also an important part in rocks; for about half the material of the entire earth's crust is oxygen. Another chief material in rocks is _silicon_. This makes up one-quarter of the crust, leaving only one-quarter to be accounted for. Silicon mixed with oxygen makes silica or quartz. There are few rocks which have not a large amount of quartz in them. Common flint, sandstones, and the sand of our shores, are made of quartz, and therefore belong to the first class of Silicious or Flint Rocks. Granites and lavas are about one-half quartz. The beautiful stones, amethyst, agate, chalcedony, and jasper, are all different kinds of quartz. Another chief material in rocks is a white metal called _aluminium_. United to oxygen it becomes alumina, the chief substance in clay. Rocks of this kind--such as clays, and also the lovely blue gem, sapphire--are called Argillaceous Rocks, from the Latin word for clay, and belong to the second class. Such rocks keep fossils well.