Classification of Massive Rocks. 295 
SYNOPSIS OF ROSENBUSCH’S NEW SCHEME FOR 
THE CLASSIFICATION OF MASSIVE ROCKS. 
BY W. 8S. BAYLEY. 
Ill. Tue Errustve Rocks. 
: bo effusive or volcanic rocks are those which flowed out upon 
a land surface and there solidified. For most of the intrusive 
rocks there are corresponding effusive ones, as might naturally be 
expected, but these latter are usually slightly more acid than the 
former. ; 
The characteristic structure for this group is the porphyritic. 
The cooling of the effusive rocks takes place in two stages, (1) while 
the rock mass is still within the depths of the earth—the intratel- 
lurial period, and (2) after it has flowed out upon the surface—the 
Gusive period. During the first stage certain minerals crystallize 
from the magma. These are idiomorphically developed, and 
become the porphyritic crystals in a ground mass which is produced 
by the cooling of the residual magma after it has reached the sur- 
face. As the cooling during the effusive period is comparatively 
rapid, there is a tendency for the ground mass of this group of rocks 
to approach the glassy condition. When, however, the cooling in 
the effusive period takes place slowly enough to allow of complete 
crystallization, a holocrystalline ground mass results and the rock 
assumes a holocrystalline-porphyritic structure. 
When the cooling is rapid the ground mass is glassy, and the rock 
is said to have a vitrophyric structure. Between these two extremes 
are other rocks whose ground mass is composed partly of crystalline 
minerals and partly of glass. This is the hypocrystalline-porphyritic 
structure, 
. The ground mass of the holocrystalline-porphyritic rocks may be 
80 developed as to possess either a hypidiomorphic-, a panidiomor- 
phic- or an allotriomorphic-granular structure. 
Since the structure of the older members of the effusive rocks 
