(2) _Iron-ore Group._—These interesting and important stratified rocks include the three principal oxides of iron,—limonite, hematite, and magnetite,—as well as the carbonate of iron, siderite; and the rocks have essentially the same characteristics as the minerals. In economical importance they are second only to the coals; and the history of their formation through the agency of organic matter is one of the most interesting chapters in chemical geology (see page 26). The three oxides are easily distinguished from each other by the colors of their powders or streaks, and the magnetism of magnetite, and from all other common rocks by their high specific gravity. Magnetite is the richest in iron, and limonite the poorest. As regards the degree of crystallization and order of occurrence in the formations, they form a series parallel with the coal series, thus:— Limonite, never crystalline, and found in recent formations. Hematite, often crystalline, and found in older formations. Magnetite, always crystalline, and found in oldest formations. Siderite effervesces with strong acid; and this separates it from all other rocks, except limestone and dolomite; and from these it is distinguished by its high specific gravity. As a mineral, siderite is often light colored; but as a rock it is always dark, and usually black, from admixture chiefly of carbonaceous matter. In studying dynamical geology, we have learned (page 28) the reason for the intimate association of siderite with beds of coal, and this accounts equally for the carbon contained in the rock itself. The connection of this rock with the coal-formations adds much to its value as an ore of iron. Finally, the iron-ores, at least where of much economical importance, are truly stratified. This can often be seen in hand-specimens; and is well shown by their relations to other rocks, in quarries and mines; and in many cases, for limonite and hematite, by the fossils which they contain. (3) _Siliceous Group._—These rocks are composed of pure silica in the forms of quartz and opal. When first deposited, whether organically, like tripolite, or chemically, like siliceous tufa, the siliceous rocks are soft and light, and the silica is in the form of opal. Subsequently it changes to quartz, and the rocks assume the much harder and denser forms of chert and novaculite, respectively.