Classificution of Massive Rocks. 209 
the latter to assume a form which it would not do were it free to 
obey the forces at work within itself, tending to bound it with 
certain definite crystal planes. 
A mineral is described as occurring in but one generation in a 
rock when all of its individual members have separated out con- 
tinuously in the same interval of the rock’s formation. When a 
portion of the individuals have separated out during one interval, 
and then, after other minerals have crystallized, another portion 
has separated, the mineral is said to occur in two generations. 
When the constituents of a rock occur in but one generation, the 
rock is granular in structure. When but a small portion of these 
are idiomorphically developed, the rock is Aypidiomorphically 
granular. When a relatively large portion or all of the constitu- 
ents are idiomorphically developed, the rock is panidiomorphic, 
When none of the constituents are so developed the structure is 
allotriomorphically granular. 
A porphyritic rock is one which contains one or more minerals. 
in more than one generation. 
I. Inrrustve Rocks. 
The intrusive or plutonic rocks are those which were formed at 
great depths. They were intruded between other rocks which 
existed before them, either as bosses or sheets, which never reached 
the surface, or they are the deep-seated portions of large masses. 
which may have flowed out upon the surface of the earth. They 
may have been formed at any geological age, but are only found in 
the oldest portions of the globe, because only in these portions has 
sufficient time elapsed to allow of their exposure by erosion. 
They are characterized by the possession of a hypidiomorphie 
granular structure, although in certain cases, where these rocks 
were intruded as flows between others, they sometimes tend to the 
Panidiomorphic development. 
They are divided, according to their chemical and mineralogical 
Compositions, into eight families. 
A. THE GRANITES. 
The granites are composed essentially of quartz and an alkaline 
feldspar, and one or more of the minerals of the mica, amphibole 
or PyToxene groups, sometimes tourmaline, and almost universally 
certain apatite, zircon and iron oxides. 
