1 1 6 The American Geologist. August, \m< 
laid ofif on vertical and horizontal axes according to their 
value. Here the relative values of Ca, Na, and K serve the 
same purpose. He considers two magmas in which potash is 
in excess, and also two each for an excess of lime and soda. 
The elements silica, alumina, magnesia and iron are represent- 
ed by the vertical lines by which their values are expressed. 
The author prefers a different scheme of representing the 
comparative analysis of massive rock. This scheme and its 
application to a large number of analyses comprise the chief 
part of this work. 
In the first place he recognizes but two eruptive magmas 
having sufficient individuality to enter into a fundamental dis- 
cussion. All others he considers arbitrary, accidental or fan- 
ciful. These two are: 
(i) That w^iich is destitute of ferromagnesian elements 
and wdiich Rosenbusch has divided into two parts, viz: foyalitic 
and granitic. It is essentially an alkaline magma, from a 
chemical point of view, and contains no magnesia. 
(2) That which is almost exclusively composed of ferro- 
magnesian elements, comprising lamprophyres and perido- 
tvtes, corresponding to the peridotic type of Rosenbusch and 
to a part of that of his gabbros. Chemically this contains a 
larger amount of magnesia than of lime. 
Thus magnesia is taken as the point of departure in his 
classification. It represents, with the greatest fidelity, the 
relative quantity of the ferromagnesian magma. Iron is more 
or less irregularly introduced, lime plays a double role, being 
partly involved in the feldspars and partly incorporated in the 
bisilicates or mono-silicates. Magnesia is always character- 
istic of the ferromagnesian elements, bisilicates, monosilicates, 
black micas, etc.. and is present in inverse proportion to 
silica. 
Secondly the author considers the relation existing be- 
tween the alkalies (potash and soda) as the only characteristic 
and reliable trait on which to base the division into families. 
This is deducible from the study of Mr. Iddings of rocks of in- 
creasing acidity (which increases as the magnesia diminishes) 
in which it is seen that everything else in liable to irregular 
variation. It is an error to complicate the relation of the 
alkalies by a comparison with the total lime, which tends to- 
ward zero with the ferromagnesian elements. 
