296 Classification of Massive Rocks. 
presents features which are different from those presented by the 
younger ones, it is convenient to separate them into an older and a 
younger group. The beginning of the Tertiary age seems natu- 
rally to be the line of division (in time) between the two groups. 
III. A. THe PALAovoLcanic ErrustvE Rocks. 
The paleovoleanic division of the effusive rocks is distinguished 
macroscopically by the lithoidal or stony character of the ground 
mass of its component members. | 
It includes a continuous series of rocks corresponding in miner- 
alogical composition to the series composing the intrusive class, 
and, like this latter, is divided into families in accordance with the 
nature of the principal constituent minerals, 
A. THE QUARTZ—PORPHYRIES. 
The quartz porphyries are the old effusive equivalents of the 
granites. By far the larger portion corresponds in miner- 
alogical composition to the granitites, so that no attempt has been 
made to subdivide them on mineralogical grounds. 
They are porphyritically developed—quartz, orthoclase, biotite, 
sometimes hornblende and augite occurring in porphyritic crystals. 
The difference of structure observed in their ground mass affords the 
basis for their further subdivision. 
1, MickoGRANITE, in which the ground mass is a very fine panidi- 
omorphic- or hypidiomorphic-granular combination. 
of quartz and orthoclase. 
2. GRANOPHYRE, in which the ground mass is holocrystalline, but 
is composed of quartz and orthoclase, developed in 
such a manner as to mutually penetrate each other. — 
3. FELSOPHYRE, in which the ground mass is so very fine that its 
components cannot be recognized under the micro- 
scope. When carefully examined it appears as an 
almost isotropic substance with, however, some indi- 
cations of structure. 
4, VrTROPHYRE, in which the ground mass is a glass, with or with- 
out microlites and devitrification products. 
(A) pitchstone porphyry contains porphyritic crystals, 
recognizable by the naked eye. 
(B) pitchstone contains no macroscopic porphyritic-crystals. 
