838 THE JOURNAL OF GEOLOGY. 



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 chemi- 

 cal 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, 

 aenigmatite 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 amphi- 

 boles. 



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 per- 

 centages of silica, may grade through similar ranges of silica, 

 but may each embrace different kinds of rocks. Thus : 



Silica Silica 



Percentages. Yellowstone Park. Percentages. Vesuvius and Ischia. 



48-53 Basalt. 46-55 Leucitophyre. 



, ( Pyroxene-andesite. 



( Hornblende-andesite. 55 _ 62 Trachyte. 



fS/t-A8 \ Hornblende-mica-andesite. 



4 ( Dacite. 



70-75 Rhyolite. 69-71 Rhyolite. 



In such series it happens that rocks bearing the same name 

 differ in certain mineralogical respects, and are really more 



