366 WHITMAN CROSS 
The ‘Rother Gneiss’’ corresponded to Bunsen’s ‘normal 
) 
trachytic”’ magma, and the augite-porphyry to the ‘normal 
pyroxenic”’ magma. 
The primary divisions of Scheerer are obviously very nearly 
the same as Durocher’s acid, hybrid, and basic groups. The 
types themselves seem to have been established under the con- 
trol of a genetic theory, and contrary to the evidence afforded 
by the tables of rock analyses published by Roth three years 
before the appearance of Scheerer’s proposition. The latter does 
not refer to the conclusions reached by Roth, which were so 
directly opposed to his own. 
Ferdinand Zirkel, 1866.—Shortly after the works by Roth, 
von Cotta, and Scheerer there appeared (1866) a work which 
may be taken as well representing the stage of development of 
petrographic system in Germany at the beginning of what may 
be called the era of the microscope. This work is the Lehrbuch 
der Petrograplie, by F. Zirkel, who was to be one of the master 
spirits of following decades. 
This treatise was, and, indeed, still is, a mine of useful 
information to the student, and its contribution to the systematic 
science was of much influence. Zirkel’s elementary scheme of 
classification is the following: 
A. Original crystalline rocks ( “ Urspriingliche krystallinsche’”’ ). 
I. Simple rocks ( ‘‘ Einfache Gesteine’’ ). 
II. Composite rocks ( ‘‘ Gemengte Gesteine”’ ). 
1. Composite crystalline-granular and porphyritic rocks (‘‘ Gemengte 
krystallinisch-kérnige und Porphyr-Gesteine”’ ). 
2. Composite crystalline-schistose rocks (‘‘ Gemengte krystallinisch- 
schieferige Gesteine”’ ). 
B. Clastic rocks ( ‘“ Klastische Gesteine”’ ). 
This arrangement has elements drawn from various sources. 
In the grand division into original and clastic (secondary) Zir- 
kel followed Naumann, but returned to von Leonhard for the 
criterion of the second order. In rejecting von Cotta’s system, 
based on mode of origin, Zirkel remarked that this plan would 
be highly satisfactory were it not founded upon hypothetical 
considerations. He secured divisions closely corresponding to 
