Page:The Journal of geology (IA journalofgeology11893univers).pdf/418

viscous, and crystallization generally takes place, but the two are in a measure independent operations, and the viscosity may be advanced so rapidly that crystallization is more or less completely prevented and glassy rocks result. According to the conditions under which rock magmas cool solidification will be accompanied by more or less complete crystallization. The size also of the crystals will vary with the rate of cooling, and the general texture of the rock will be affected. Different parts of one rock magma may experience different conditions of cooling, and there will result a variety of textures or structures within the mass. It may be that the textural differences are sufficiently pronounced to be given distinctive names, which become the terms by which certain kinds of rocks are designated; for example, granite, porphyry, pearlite, pumice, etc. There is then a relationship between certain kinds of igneous rocks which exists because of different conditions which have attended the solidification of various portions of one body of magma, or of several magmas alike in other respects. The significance of this relationship was long ago appreciated by James D. Dana,^[1] who maintained that the textural differences among rocks were mainly due to the physical conditions under which they consolidated; an idea ably advocated and corrobated by Judd,^[2] and more recently substantiated by numerous observations in many localities.

Igneous rocks often differ from one another in mineral and chemical composition; in fact, some kinds differ so widely from one another in a mineralogical sense that they possess no mineral in common. And most kinds contain the minerals which may be common to them in quite diverse proportions, and associated with various other species. Chemically they consist of the same essential constituents in variable proportions, the variations being within certain limits. But the proportions are so far from being