Sulphate of Lime, or Gypsum, is composed of sulphuric acid and lime. Its specific gravity is 2.32. When crystalline, it has a pearly lustre, is transparent, and goes under the name of Selenite. Common Gypsum resembles the other earthy limestones, but it is softer, and may be readily distinguished by its not effervescing with acids.
To the minerals now enumerated may be added the following, which are of frequent occurrence, but not in great quantities; namely, carbonate of magnesia, oxide of iron, iron pyrites, rock-salt, coal, bitumen, schorl and garnet.
These simple minerals, either in separate masses or mingled more or less intimately together, compose almost wholly the earth’s crust.
SECTION III.—THE MINERAL MASSES WHICH FORM THE CRUST OF THE EARTH.
That portion of the structure of the earth which is accessible to man is called the crust of the earth.
The mineral masses which compose it, whether in a solid state, like granite and limestone, or in a yielding state, like beds of sand and clay, are called rocks.
The unstratified rocks are Granite, Hypersthene rock, Limestone and Serpentine, and the Trappean and Volcanic rocks.
Granite is a rock of a light gray color, and is composed of quartz, felspar and mica, in variable proportions, confusedly crystallized together. The felspar is generally the predominant mineral. It is sometimes of a very coarse texture, the separate minerals occurring in masses of a foot or more in diameter. At other times it is so fine-grained that the constituent minerals can scarcely be recognized by the naked eye; and between these extremes there is every variety. The term granite is not, however, confined to an aggregate of these three minerals. In some instances the felspar so predominates as almost to exclude the other minerals, when it is called felspathic granite. When the quartz appears in the form of irregular and broken lines, somewhat resembling written characters, in a base of felspar, it is called graphic granite. When talc takes the place of mica, it is talcose granite. When hornblende takes the place of mica, it is syenite. Granite or any rock becomes porphyritic when it contains imbedded crystals of felspar.
There is a rock of crystalline structure, like granite, but of a darker color, which is called hypersthene rock. It is composed of Labrador felspar and hypersthene. The mineral species serpentine and limestone often occur unstratified in considerable quantities.
Volcanic rocks consist of the materials ejected from the craters of volcanoes. They are composed of essentially the same minerals as trap rocks. When the material has been thrown out in a melted state, it is called lava. Lava, at the time of its ejection, contains a large amount of watery vapor at a high temperature. Under the immense pressure to which it is subjected in the volcanic foci, it may exist in the form of water; but when the lava is thrown out at the crater, the pressure cannot much exceed that of the atmosphere. The particles of water at once assume the gaseous form. As lava possesses considerable viscidity, the steam does not escape, but renders the upper portion of the mass vesicular. This vesicular lava is called scoriæ. By the movement of the stream of lava, these vesicles become drawn out into fine capillary tubes, converting the scoriæ into pumice-stone.