368 WHITMAN CROSS 
shown to be erroneous, that four kinds of feldspar, namely, 
orthoclase, oligoclase, labradorite and anorthite, seldom occurred 
together, and therefore might be used to characterize four great 
series of rocks. The feldspathoids were used to define another 
large series. 
It appears that in this proposition to use qualitative mineral 
composition as a leading factor in classification, as in that to 
apply the age distinction, Zirkel merely gave definite expression 
to the growing usage of the time, which was practically found 
in other systems, though not so clearly avowed as a principle. 
The effect of his proposition to apply geological age and this 
qualitative element of mineral composition as leading factors in 
the systematic arrangement of the rocks we now term igneous 
was peculiarly unfortunate, because this invaluable work of refer- 
ence was issued at the beginning of the era in which petrogra- 
phers were to be so busily engaged in the microscopical study of 
rocks that they had no time for systematic work. In the flood 
of descriptive literature of the succeeding decade these proposi- 
tions were adopted almost of necessity. The students from all 
lands who flocked at this time to Germany to study under Zirkel 
and other masters, carried the system back to their respective 
countries, giving it quickly a world-wide usage. 
Ferdinand von Richthofen, 186S5.—Shortly after the appear- 
ance of Zirkel’s Lehrbuch a philosophical discussion of the clas- 
sification of igneous rocks, as viewed from the geologist’s stand- 
point, was published by the distinguished German traveler and 
geologist, Ferdinand von Richthofen. This essay was written 
and published during an extended visit in the United States, 
under the title ‘Principles of the Natural System of Volcanic 
Rocks.’”? 
Von Richthofen calls systematic petrography ‘‘the most 
intricate branch of descriptive natural science.”’ He character- 
izes the earlier systems as artificial, because based upon the idea 
“that classification should be made dependent on one certain 
principle previously assumed as the point of issue.” The prin- 
* Memoirs presented to the California Academy of Sciences, Vol. 1, pp. 39-133, 1868. 
