466 THE AMERICAN NATURALIST.  [Vot. XXXII. 
relationships are expected to be shown in it. He believes “that no 
great progress in systematic petrography is possible until a more 
rational view of the relationship of that science to geology prevails 
among its devotees.” The rock, in petrography, is a unit of material; 
while in geology it is a unit of form or mass. The geological rock 
is the subject of study in fetrology. The classification of rocks 
in petrography should be a classification based on facts and not 
on theories; it should be based on the properties of the rocks them- 
selves, and upon their relationships to one another and to the earth. 
No natural classification of rocks is possible, because of the nature 
of these bodies. ‘‘The systematic classification of rocks, according 
to which their specific names are applied, must be based on their 
properties as objects, together with only such geological criteria as 
may be found adaptable, to the end that the system may be uniform, 
stable, and as natural as possible.” The author examines critically 
the accepted scheme of classification as now used, and shows that it 
is illogical, being based primarily on geological criteria that are 
largely theoretical. He objects also to the founding of the classifica- 
tion upon such hypothetical factors as those embraced in the theory 
of magmatic differentiation. On the other hand, “the material 
properties of igneous rocks afford ample criteria for establishing a 
systematic classification.... Since the geological factors of age, or 
of form, or place of occurrence, are not directly causes of the prop- 
erties used in classification, they cannot be applied to produce 
coordinate groups.” 
Leucite Rocks from Montana. — In another paper Cross! reports 
the existence of a most interesting series of leucite rocks at the 
Leucite Hills and Pilot Butte, Wyoming. Some of these rocks have 
already been described by Zirkel, Emmons, and Kemp, but none of 
these geologists had learned of the great variety of types in the 
region. The principal area of leucite rocks is a mesa whose top 
consists of a surface flow of porous and massive rock material, the 
latter of which corresponds to Zirkel’s leucitite, while the vesicular 
rock is a sanidine-leucite aggregate. The massive rock is redescribed 
by Cross as consisting of phlogopite crystals in a groundmass made 
up of leucite crystals and anhedra, separated from one another by 
pale green or colorless microlites of diopside, imbedded in a very 
siliceous glass. This rock the author calls wyomingite. 
The principal rock of the Leucite Hills is the sanidine-leucite 
1 Amer. Journ. Sci., vol. iv, p. 115, 1897. 
