Classification of Rocks. — Spurr. 225 
point of view; for example, in the porphyric rocks one must not sacri- 
fice the elements of the second period of consolidation to those of the 
first. 
The General Principles of the Classification of the Feldspar Rocks. 
Feldspar rocks were first divided according to whether the 
feldspar was monoclinic or triclinic — orthoclase or plagioclase. 
This classification has left its lasting influence upon us and is 
still in full use by many. Btit the most advanced petrogra- 
phers now speak no longer of the orthoclrse rocks as con- 
trasted with the plagioclase rocks, but of the alkali feldspar 
rocks as opposed to the soda-lime feldspar rocks. This di- 
vision of the feldspars is used as the prime mineralogical basis 
for separating the rock families, by Rosenbusch, Zirkel, 
Michel-Levy, and others. 
It is in the soda-lime feldspar rocks that there is the most 
confusion, as everybody has felt. One has only to consider 
how indefinitel}' terms like dior\'te and porphyr^'te (concern- 
ing the latter of which Michel-Levy trul)' remarks that it com- 
prises one-half or one-third of the natural rocks) have been 
used; and to consider the overlapping of terms like dioryte, 
diabase, and gabbro, of which sometimes one and sometimes 
another is tised for the same rock. In fact, the term plagio- 
clase rocks has been a sort of waste basket. 
Rosenbusch separated the plagioclase rocks into separate 
families by means of the ferromagnesian constituents. He 
considered dioryte as characterized, besides plagioclase, by the 
ferromagnesian minerals amphibole or mica, while diabases 
and gabbros were characterized by pyroxene and olivine. The 
volcanic rocks were also divided by a similar movable and in- 
definite line into andesytes and basalts, hornblende and mica 
characterizing the former^ and pyroxene and olivine the 
latter. 
Fouque and Michel-Levy separated the Tertiary and post- 
Tertiary trachytoid rocks, containing biotite, amphibole, or 
pyroxene, into two classes on the basis of the si^ecics of the 
feldspars, calling those containing oligoclase-andesine, ande- 
sytes, and those containing labradorite, labradorytes, and 
omitting from this classification those containing anorthite. 
.Ml the soda-lime feldspar rocks containing olivine they classi- 
