Page:The Journal of geology (IA journalofgeology11893univers).pdf/356

so radical a change in chemical composition. These inclusions must, by this theory, be considered to be fragments of older rocks, formed in this way. Still basic inclusions may be supposed to have been formed by mechanical agglomeration, and no doubt this has often been the case. But, in opposition to both these theories, it is in many cases evident that the inclusions were soft, and then the simplest view is that they were drops, or portions, of a partial magma, which at the temperature, existing immediately before crystallization, could no longer be held in solution by the principal magma, but separated out.

The great petrographical province of Iceland is characterized principally by enormous eruptions of plagioclase-basalts and exceedingly subordinate eruptions of rhyolites, which, however, are very numerous. No other eruptive rocks are known from Iceland up to this time.^[1] If we considered the differentiation of the primary magma, which here was very basic, as a diffusion-phenomenon, according to "Soret's principle," it would be incomprehensible why the differentiation never stopped with the production of an intermediate magma, and, moreover, this theory would demand that every little rhyolite-magma previous to the eruptions would have been surrounded by a broad zone, showing all transitions to the basaltic magma. In both cases these intermediate magmas should have been erupted at some time, but, as already mentioned, we know a hundred eruptions of rhyolite but not a single one of andesitic rocks. It therefore seems more probable that these intermediate magmas never existed in the petrographical province of Iceland, but that the acid partial magmas were separated out directly from the basic original magma, which by lowering temperature lost its homogeneity. The conditions of temperature and pressure being different in different places these acid partial magmas also became somewhat different, but may all be classified as soda-rhyolites. The chemical compounds, which constitute the silicate magmas—and which are not necessarily identical with the rock-forming