SYS IL SNAA IL. SI APIS SOC A NC 367 
von Cotta’s eruptive, sedimentary, and metamorphic classes by 
roundabout means. 
In his arrangement of original simple rocks, Zirkel practi- 
cally followed Naumann. 
In the Lehrbuch igneous rocks were given the greatest atten- 
tion. They were brought together in one group by applying 
three factors at once, as composite crystalline-granular and porphy- 
yitic rocks. \n practice the noncrystalline or vitreous rocks 
were also included here. This inconsequent proceeding was 
necessary to bring the eruptive or igneous rocks together, a fact 
demonstrating that the chosen method of primary subdivision 
was logically incorrect. The first subdivision of this group was 
by relative age into the Older and Younger rocks, a distinc.ion 
which Zirkel himself recognized was highly artificial.* He 
pointed out that rocks of certain characters had actually received 
different names, according to age, although the time factor had 
not been used in their systematic arrangement ; and he chose to 
be logical in application of that factor, to agree with usage, 
rather than to eliminate the duplicate terms. Had he chosen 
the latter alternative it is safe to assert that much of the still 
existing confusion from unnecessary duplicate rock names might 
have been avoided. me 
In further systematic subdivision mineral composition and 
texture were used in ways which have been followed more or less 
closely by many petrographers to the present time. Following 
Roth and others it was first determined to consider all rocks of 
the groups in question as feldspathic or non-feldspathic. Felds- 
pathic rocks were held to include all in which feldspars or felds- 
pathoids were present in appreciable amount. By this course 
two very unequal divisions were created and the qualitative ele- 
ment of mineral composition was confirmed in its position of 
dominance over the quantitative, which has had such unfortunate 
influence upon systematic petrography to this time. 
The plan of forming a great group of feldspathic rocks may 
have appeared to be desirable on account of the belief, long since 
t Lehrbuch, Vol. 1, pp. 446, 447. 
