88 
IDDINGS. 
varieties the crystallization of the porphyritical quartz con¬ 
tinued uninterruptedly into the period of the crystallization 
of the groundmass, the substance of the quartz being the 
same in the outer portions of the individual, which have a 
granitic habit, and in the central portion, which corresponds 
to the porphyritical individuals in those parts of the rock 
where the early crystallization was suddenly checked. The 
final crystallization of this rock took place beneath the sur¬ 
face of the earth, but still at no very great depth. From such 
instances as this it is evident that the porphyritical crystals 
in many rocks were crystallizing while the magma was 
in motion, and did not cease their growth after it came to 
rest until the whole magma had crystallized. 
Relation of Rock Structures to Geological Occurrence. —(1.) The 
degree of crystallization of igneous rocks is most intimately 
related to their geological occurrence. Glassy rocks are found 
where lavas have flowed out upon the surface of the earth, 
and also where small bodies have penetrated other rocks, the 
indications all leading to the conclusion that rock glasses are 
the result of rapid cooling. 
Porphyritic, fine-grained rocks occur in the central portions 
of surface flows and in intrusive bodies, where the cooling was 
evidently slower than that which produced the glasses. 
Most of the coarsely crystalline rocks are found in large 
intrusive bodies, or in those small bodies which appear to 
have been intruded in rocks already heated, where the cool¬ 
ing must have been extremely slow. 
(2.) When the variations in the crystallization of a large 
body of rock are studied in connection with its geological 
occurrence it is sometimes observed that the variations, 
which are evidently due to local causes and can be traced to 
the immediate surroundings, affect the crystallization of all 
the minerals in the rock, proving that in these cases they 
were all crystallized in the position now occupied by the 
rock. Thus, in the case of a diorite that crystallized within 
a few thousand feet of the surface of the earth in cretaceous 
