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another and to the other constituents is characteristic of the rocks of each group. A genetic relationship is clearly indicated, and it appears that the various rocks in each locality have been derived from a general magma peculiar to the locality.

The distinguishing characteristics of the rocks of different petrographical provinces which may be observed in their chemical composition also find expression in certain mineralogical peculiarities. Thus the presence of a relatively high proportion of potash will insure an abundance of potash-bearing minerals, as at Vesuvius. The relatively high percentage of soda in the rocks of Pantellaria, together with low alumina and relatively high ferric oxide, determines the prevalence of alkali-feldspars rich in soda, and of soda-bearing ferro-aluminous silicates, ænigmatite or cossyrite. The less prominent position of the alkalies in the rocks of Electric Peak and Sepulchre Mountain, and the relatively higher percentages of magnesia and iron oxide leads to the very general presence of orthorhombic pyroxene in these rocks, which is in contrast to the less magnesian and more alkaline rocks of Central France and Germany. The abundance of alkalies and general preponderance of soda in the rocks of the Christiania district expresses itself in the abundance of the alkali-feldspars and feldspathic minerals, and in the prevalence of acmite- and riebecite-molecules in the pyroxenes and amphiboles.

From this it follows that certain rocks belong in particular natural series or groups, and are absent from others, and that two natural series of rocks, when arranged according to the percentages of silica, may grade through similar ranges of silica, but may each embrace different kinds of rocks. Thus:

In such series it happens that rocks bearing the same name differ in certain mineralogical respects, and are really more