2i6 The America?i Geologist. April, i9no 
the variety of the minerals, and second, of the quantitative 
amount. As an example of the second principle, actually ap- 
plied in the classification, may be mentioned the granite group 
and the quartz-syenyte group, each consisting of alkali feld- 
spars, quartz and any or all of the principal ferromagnesian 
silicates ; the quartz-syenyte differs from the granite in having 
only subordinate or accessory quartz, while that of the granite 
is of prime importance. 
In establishing groups the ferromagnesian minerals, with 
the exception of olivine, are reckoned together, following 
many good authorities*. These ferromagnesian minerals 
occur throughout the rock series from one end to the other 
without excluding one another '. The predominance of one 
ferromagnesian constituent over the others is recognized in the 
divisions of the groups, or in the rock species, and this rock 
species is designated by writing the characteristic ferromag- 
nesian silicate before the name of the group, such as pyroxene 
dioryte, amphibole syenyte, etc. Subdivisions of the rock- 
species, or rock varieties, are formed by the predominance 
of some special variety of one of the ferromagnesian silicates, 
and these subdivisions or varieties are designated in the same 
way. Under the species pyroxene dioryte, for example, there 
occur the varieties augite dioryte and hypersthene dioryte. 
The mineralogic class, family, group, species, and variety 
being thus established, the type, which is the actual designation 
of any given specimen, depends upon the structure. It is a 
well acknowledged fact in general that rocks of different 
structure may have the same chemical and mineralogic com- 
position and that their textural differences are due only to dif- 
ferent conditions of consolidation, which are often slight;|;. The 
chief structures have been divided into five : First, the granu- 
lar, hypidiomorphic or allotriomorphic structure. The rocks 
characterized by this structure are given the general name as 
used in the mineralogic classification: thus, granite, dioryte, 
olivine -diabase, etc. Second, the panidiomorphic structure, 
which is generally fine-grained, sometimes coarse. This is 
*Zirkel. op. cit., p. 645. 
tPrincipes a suivre pour une Classification universelle des Roches, 
par M. A. Michel-L^vy, p. 156. 
JZirkel, op. cit., p. 640. 
