Lcivi/iso/i-Lcssifigs Classification of Rocks. 349 
this: that the alkalies predominate in the magmas of the el- 
eolitic syenites, which are more acid than the basalts, in which 
the alkaline earths predominate. 
III. Neutral rocks: Absence of olivine, nepheline, of 
leucite ; predominance of the bisilicates and the most acid feld- 
spars with sometimes a small amount of free cjnartz. The rocks 
of this group have nearly all the same formula. The coeffic- 
ients(c»') have also very nearly the same value. The relation 
R'.;0:RO alone distinguishes them. Lewinson-Lessing as- 
cribes them to metamorphism of earlier rocks. 
IV. Acid rocks: Great quantity of free quartz; predomi- 
p.ance of the most acid feldspars, and a relatively feeble role of 
the bisilicates. i. e. of mica; for coefftcient of acidity the limits 
are from 2.5 to 4.5 or 5. In certain keratophyries <ar runs from 
6 to 7 but in that case the author supposes there has been a de- 
position of free secondary cjuartz. Certain petrographers, Ros- 
enbusch for example (and Lewinson-Lessing formerly), con- 
sider an acid magma to be a neuter or basic magma with added 
quartz- ^. ^., granite equals s)'enite+quartz; quartz-dioryte 
equals dior)'te-t-quartz. That is correct from a mineralogical 
point of view, but fault)- from a chemical point of view. Gran- 
ite, for instance, is not alone richer in SiO^ than s>'enite, but 
the relations of R=0;RO and RO rR^Os are also different. 
Relations bctiveen Different Oxides a?id Groups of Oxides. 
Are there distinct types of fundamental magmas? Rosen- 
busch replies yes to this question and distinguishes five or six 
principal types. Lewinson-Lessing distinguishes many more 
— but before entering upon that subject it is appropriate to ask 
what should be the distinguishing characters of these princi- 
pal magmas. Lewinson-Lessing considers as a distinctive sign 
of a chemical type of a magma "an independent difference, " 
but not a parallel difference, so to speak. For example, if in 
two groups of rocks, one more rich in a constituent part a 
and the other poor in a there is the same relation toward a 
constituent part b. The differences are parallel, as used by the 
author. 
/ Group II Grorp 
Differences ) m. a vl a and m>n 
parallel and > 
•direct. \ x^ zb and x>z 
