208 Classification of Massive Rocks. 
slowly under great pressure and at great depths, where crystalliza- 
tion was gradual and undisturbed, assumed a granular aspect. — 
Those which cooled quickly under low pressure became glassy. 
Those which began to crystallize in the depths of the earth, and 
continued their crystallization after the transference of their entire 
mass to other places, took on a porphyritic habit. Since, then, the 
structure of a rock indicates with some degree of certainty the 
prevailing conditions under which it was formed, it affords a conve- 
nient basis for the foundation of rock classification. And further, 
since the conditions under which a reck is formed are directly 
connected with its geological relations to surrounding rocks, the 
most logical classification is that which takes primarily into consid- 
eration these relations as the causes which produce the effects noted 
as structure. 
Rosenbusch begins, then, by separating all massive rocks into 
the three great groups mentioned above. The intrusive or plutonic 
rocks are those which formed at great depths (Tiefen-gesteine); the 
effusive or voleanic rocks are those which flowed out upon a land 
surface and there solidified (Erguss-gesteine), and finally the second 
group, the vein rocks, are those which have been found only in 
veins or dykes in other rocks, and which may or may not be con- 
nected with the effusives. 
Before discussing the classification in detail it will be necessary 
to define a few terms introduced by Rosenbusch to facilitate the 
description of the more prominent structures characterizing rocks, 
as we find them to-day. 
A rock composed akon. of i minerals is said to be 
holocrystalline. When it consists entirely of an unindividualized, 
structureless mass, it is known as amorphous. When it is partly 
amorphous and partly crystalline, ¿.e., contains crystals in a hyaline 
ground-mass, the structure is described as hypoerystalline. 
An idiomorphic mineral is one whose form is determined by the 
crystallizing forces acting within itself. An idiomorphic mineral 
is bounded by crystal planes. An allotriomorphic mineral is one 
which possesses a form due to the action of external forces, and not 
to the action of intramolecular forces. An allotriomorphic mineral 
is not bounded by its own crystal planes. Of two contiguous 
minerals in a rock, one idiomorphically developed, and the other 
allotriomorphically developed, the former is the older, compelling 
