228 The American Geologist. April, 1900 
classification into two classes necessitates the provision of a 
transitional family between these two, corresponding to the 
nionzonytes between the alkali feldspar rocks and the acid 
soda-lime feldspar rocks. This is the belugitic family, and the 
l)elugyte hr.s the corresponding porphyritic form with fine- 
grained or aphanitic grovnidniass, aleutyte, in accordance with 
the general scheme of the classification. 
As far as the system of grouping and subdivision goes, it 
is hoped that convenience will recommend it. In this classi- 
fication the class, family, group, species, variety, and type are 
carefully subordinated one to another, so that a rock may be 
variously defined, with a greater or less degree of accuracy, 
as belonging to any one of these divisions. Thus augite 
syenyte aplyte is a type name finally defining a rock. Outside 
of this the rock belongs to the mineralogic variety augite 
syenyte. and the mineralogic species pyroxene syenyte : 
further, it belongs to the syenyte group, and the granitic 
family, and in the widest sense to the class of feldspar rocks. 
".A. rational petrographic system," says Brogger*, "must, 
therefore, also soon begin to define more sharply which names 
must become fixed as class names, order names, group names, 
and names of higher order in general, and which names as 
family names, species names, and variety names." 
Structural Variations or Types. — In making structural va- 
riations subordinate to mineralogic ones the writer has fol- 
lowed the trend of development, for the tendency has been 
expressed in all modern petrography with greater and greater 
clearness. This principle of subordination has been essentially 
followed by Michel-l.ev)- and Teall and has been expressed 
more clearly in Brogger'st table, and by Turner ;|;. 
Number of Structures Afifecting Nomenclature. — Brogger§ 
remarked that at least three structures must be recognized -in 
rock nomenclature. Rosenbusch, Michel-Levy. Zirkel. and 
others generally, used three structural terms which have been 
best expressed by Michel-Levy as granitoid, porphyric, and 
*Die Gesteine der Grorudit-Tinguait Serie, Kristiania 1894, p. 96. 
tEriiptionsfoIge der Triadischen Eriiptivgesteine bei Predazzo in 
Siidtyrol, Kristiania. 1895, P- ^o. 
^Seventeenth Annual Rep., U. S. Geol. Survey, Part I, p. 719. 
^Die Gesteine der Grorudit-Tinguait Serie, Kristiania, 1894, p. 97. 
