The Splitting or Cleavage of Magmas (Spaltung, Magmatic Segregation, Mag made Differentiation).— When a magma cools, the first minerals to appear are the ores (with apatite, titanite, zircon, etc.) ; following these,: the basic ferromagnesian constituents; their the more basic plagioclase feldspars, followed or overlapped by the orthoclase varieties; and last. of all, quartz, if more than enough exists for the formation of the silicates. Obeying the principle of diffusion, the first minerals to form: the more basic ones — tend to accumulate on the cooling walls, that is, at the periphery of the magma-basin; and the other minerals ar range themselves rudely in concentric zones, each zone toward centre being successively more acid, until at the centre the magma becomes comparatively acid and may, after solidification, consist largely of such minerals as orthoclase and quartz. This arrangement of the minerals by diffusion before crystallization results in rude separation of an originally uniform magma into several magmas of different chemical com positions, which, on cooling, furnish rocks of different mineralogical compositions. The proc ess is termed the cleavage of magmas.
Definitions depending upon internal char acteristics will next be considered.
Texture.— The fluid magmas of igneous rocks may be compared to molten glass. If the cooling period be long the individual minerals form comparatively large crystals. If it be short' the crystals are correspondingly small. If it chills so suddenly that the molecules of the different mineral compounds do not have time to unite to form crystals, but are caught just as they existed originally in the fluid magma, the result 'is a volcanic glass. Rocks in which the crystals are five millimeters or more in diameter may be called coarse-grained; rocks in which the crystals range in size from one to five milli meters, medium-grained; and those in they are one millimeter or less in diameter, fine grained. Rocks in which all of the original magma has individualized or crystallized, to form minerals of some sort, and in which there is no unindividualized material remaining be hind in the form of glass, are called holo crystalline. All rocks in which the crystals are large enough to be seen with the unaided eye are termed phanerocrystalline, or phanenc. Those rocks in which the crystals are too small to be distinguished megascopically are called aphanitic. Many aphanitic rocks, however, un der the microscope are seen to be holocrystal line and to consist of small crystals of minerals which can be specifically identified with the aid of that instrument. For these rocks the term microcrystalline has been proposed. But
the crystals, though recognizable, may be too small to be specifically identified even with the microscope. Such rocks are called microcrypto crystalline. Volcanic glasses show only em bryonic crystals imbedded in textureless glass. Such rocks are said to be vitreous or glassy.
If during the entire period of solidification the conditions of cooling remained the same and the entire process of cooling of a deep seated magma was slowly and quietly accom plished in the magma-basin, unattended by any effusive period, the rock would be coarse (to fine), evenly-granular. In rocks cooling under these circumstances the growing crystals usually interfered with each other in such a way as mutually to destroy their crystal bound aries, forming irregular interlocking grains, with no one diameter much greater than the others. In other words, the grains are irregu larly rounded. A texture of this sort is char aCteristic of the granites, and is called the gran itic or granitoid texture. The term allotrimor phic, referring to the fact that the crystals do not possess their own boundaries, is applied to the same texture, and a still newer term, xeno morphic, has recently been proposed. Where the majority of the crystals do retain their crystal boundaries, the resulting texture is called idiomorphic or automorphic. If the crystal boundaries are only faintly or imper fectly discernible, the structure is said to be hypidiomorphic.
The period of solidification within the earth may be interrupted by an effusive period; in which case those crystals which had begun to form, and may have reached considerable size are carried by the eruption into other sur roundings, where the cooling process may be much accelerated. A second generation of smaller crystals would then form about the large well-formed ones, imbedding them in a fine grained producing what is termed, porphyritic texture. Two or more generations of crystals may thus be recognized. The well defined crystals of the first generation are called phenocrysts. The term felsite is applied to the fine-grained ground-mass of the acid rocks.
The crystals of some one mineral species in a rock may have one diameter much larger than the others. The crystals then appear distinctly lath-shaped in thin section under the micro scope. This form of crystallization is espe cially common with the plagioclase feldspars. These laths of plagioclase are enclosed by crystals of another species. This texture is com mon, and in the scheme of classification of igneous rocks at present accepted is called into rather prominent requisition. It is known as the ophitic texture.