Page:The Journal of geology (IA journalofgeology11893univers).pdf/417

It is desirable that the student of igneous rocks should appreciate the fundamental relationships existing between various kinds of igneous or eruptive rocks so far as they are understood at the present time, in order that he may form a proper idea not only of what an igneous rock actually is, but also of the uses and limitations of the terms by which they are designated. So it has been thought desirable to present, in an elementary form, some of the data and opinions bearing upon the genesis of different kinds of rock magmas.

It can be shown that all eruptive rock masses, whether emanating from volcanic vents at the surface of the earth or found enclosed within such vents, or confined to fissures not immediately connected with actual volcanoes, with the exception of certain infrequent occurrences of sandstones, which have been forced, while in a loose and incoherent state, into cracks—it can be shown that all ordinary eruptive masses were in a completely molten or fused condition before solidifying into the rocks they now are, and hence the terms eruptive and igneous are practically synonymous.

The igenous mass or molten magma, as we know by observations at active volcanoes, may obtain a liquidity comparable to that of water,^[1] which, of course, would obtain for different temperatures in the case of magmas having different chemical compositions; the less silicious magmas reaching this liquidity at a somewhat lower temperature than the more silicious ones. During the process of cooling, magmas become gradually more