_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_. A large part of the materials ejected from volcanoes is in the form of dust, cinders and angular fragments of rock. These soon become solidified, forming _volcanic tuff_, or _volcanic breccia_. In submarine eruptions these fragments are spread out by the water into strata, upon which other materials, not volcanic, are afterwards deposited. These interposed strata are called volcanic grits. The _trappean rocks_ are composed of felspar, mingled intimately and in small particles with augite or hornblende. They also contain iron and potassa. They are often _porphyritic_. When they contain spherical cavities, filled with some other mineral, such as chlorite, carbonate of lime or agate, they are called _amygdaloidal trap_.