266 _ . PROCEEDINGS OF SECTION C. 
may add that granite and diorite often behave in the same 
way, the one passing by easy transitions into the other in 
the same rock-mass. The United States Survey Officers 
have introduced the name granodiorite for this passage-rock. 
Syenite also graduates into diorite in a precisely similar 
manner. It would hence appear natural to group these 
three families of rocks in one great natural division. Rosen- 
busch, while adopting a totally different principle of 
classification, propounds a granito-dioritic magma, which 
corresponds with the division just suggested. 
Having marked off the rocks of this magma, there remain 
only two other great divisions, which are easily appreciated, 
namely, elaeolitic-leucitic rocks and gabbro-peridotite-pyrox- 
enite rocks. As we shall see later on, chemical considerations 
support this grouping. 
Mr. Alfred Harker, in his interesting little book, “ Petrol- 
ogy for Students,” adopts plutonic, hypabyssal, and volcanic 
as the three primary divisions, and groups the rock families 
in these in order of increasing basicity. Thus, under plutonics 
he groups granites, syenites, diorites, gabbros, and peridotites ; 
under hypabyssal rocks he ranges granite porphyries, cerato- 
phyres, diabase, lamprophyres; under volcanics, rhyolites, 
trachytes, phonolites, andesites, basalts. 
This is practically the principle followed by Rosenbusch, 
though the details are not the same, and the German 
author’s dyke-rocks are differentiation products, not merely 
hypabyssal rocks. It is really the recognition of variety 
of geological occurrence, involving variety in structural 
characters. It has the disadvantage of bringing together 
in one assemblage rock families which are widely separated 
in constitution. Hence it is, I submit, unavailable for 
primary divisions. 
The other great German petrographer, F. Zirkel, bases 
his primary divisions on mineral characters; viz.:—1. Rocks 
with dominant alkali felspar; 2. Rocks with dominant 
lime-soda and lime felspars; 3. Rocks without felspar. 
Under this grouping we have granitic and elaeolitic rocks 
thrown together in the same division. 
The French school also classifies according to the felspars. 
Though the method is empirical, the results approximate to 
the truth, for the contents and proportions of alkalies are 
involved. For example, in syenite and granite potash 
felspars are the dominant ones; in diorite, Na,O has in- 
creased ; hence, the felspar is soda-lime. In gabbro, alkalies 
are low and CaO high; hence, the felspar is lime-soda. In 
nepheline syenites, the excessive alkalies show themselves in 
all the characteristic minerals of that division. 
