298 VAN HISE—METAMORPHISM OF ROCKS AND ROCK FLOW AGE. 
erly oriented or happily placed, and thus in the first case reverse the 
tendency of mass static conditions. Thus is explained the characteristic 
uniformity in the size of the particles of the crystalline schists which have 
not been modified since mass dynamic action ceased. However, in some 
cases, where the mineral particles are properly oriented, as explained on 
page 305, the tendency for large individuals to grow at the expense of 
smaller ones may control and porphyritic textured crystalline schists be 
produced. 
The second characteristic feature of the recrystallized schistose rocks 
is that the mineral. particles show a marked tendency toward regular 
orientation. This orientation may consist in the particles having their 
major, mean, and minor diameters in approximately a common direc- 
tion; or in certain species having their crystallographic axes in a nearly 
common direction, as result of which the like cleavage of all the parti- 
cles of a certain mineral is approximately in the same plane; or in hay- 
ing the two combined. This arrangement, where marked, transforms 
the rocks into cleavable slates or schists. 
The most important of the minerals the particles of which show a 
similar crystallographic orientation are micas, and especially biotite and 
muscovite. With these minerals similarity of orientation is usual. The 
minerals next in importance which frequently show a marked tendency 
toward similar crystallographic orientation are feldspar, chlorite, and 
amphibole. Other less important minerals are known to show the same 
phenomena. Of course, it is understood that the crystallographic orien- 
tation is in no case perfect, but with mica it may approach perfection. 
From the extreme of regularity of orientation shown by mica in the typ- 
ical crystalline schists to the random orientation of some of the minerals 
in the same schist there are gradations; also there are gradations from 
the crystalline schists to rocks recrystallized under mass static conditions 
where none of the minerals show a marked HON to similar crystal- 
lographie orientation. ; 
In many cases the similar orientation of mineral particles in a typical 
crystalline schist may have been greatly disturbed by subsequent defor- 
mation near the surface, and therefore in the zone of fracture. Under 
such conditions shearing fractures may be produced parallel to the schis- 
tosity, and the shearing motion between the layers may largely destroy 
the original regularity of the oriented particles. 
Some of the mineral constituents of igneous rocks which have not been 
recrystallized show a tendency toward a parallel crystallographic orienta- 
tion.. However, with this structure are other structures characteristic of 
rocks crystallizing from a magma. I know of no instance in which an 
unaltered igneous rock so closely resembles the crystalline schists that 
there is any trouble in discriminating between them. 
