147 
The principal cause, though not the only one, is what may 
be termed “oscillation” between two distinct crystalline 
forms, a tendency towards each of which may be present 
during the formation of a crystal, but one more powerful than 
the other. 
The stronger tendency determines the general form, while 
the weaker one leaves its record in a series of attempted faces, 
which usually appear only as fine strize ; at times, however, 
and indeed almost always with a magnifier, these lines are 
seen to be the mere edges of faces belonging to a different 
form. Thus the horizontal striation on the faces of quartz 
erystals is usually due to oscillation between the prism and 
one or both of the terminal pyramids; as may be seen with 
a strong glass. Sometimes, moreover, quartz crystals plainly 
exhibit a succession of numerous pyramids, one above an- 
other; most collectors are acquainted with this peculiar 
structure, which is in fact simply an exaggerated degree of 
the familiar striation. Ordinary iron pyrites, also, is one of 
the best species in which to observe these phenomena. Here 
the two forms between which this interaction most frequently 
occurs, are the cube and the pentagonal dodecahedron,—one 
holohedral, the other hemihedral. Cubes of pyrite bear the 
striz of the pentagonal dodecahedron, the tendency toward 
the cubical form having in this case prevailed over the other. 
Conversely, when the hemihedral tendency succeeds in de- 
termining the general figure, we find the pentagonal dodeca- 
hedron bearing the strize of the cube. 
In similar ways, we find these oscillations occurring be- 
tween two hemihedral forms, as e. g., the pentagonal dodeca- 
hedron and diploid, or between two holohedral forms, as in 
some garnets, etc., between the rhombic dodecahedron and 
the octahedron or the cube. 
In the orthorhombic system, the oscillation of the basal 
and pyramidal planes is a frequent source of striation. So 
also macrodome or brachydome striz are produced on the 
basal planes by oscillation of the latter with one or other of 
the former. In monoclinic crystals, the corresponding stria- 
tions are, of course, those of the orthodome and clinodome. 
With regard to the previous paper, Prof. Egleston re- 
marked upon the close relation between the hexagonal 
and the orthorhombic systems, and the frequency of hex- 
agonal twinning in the latter. In these cases, and many 
others, striation bears a most valuable part in determining 
both the fact and the manner of twinning; and indeed it is 
often the only evidence proving such a relation. 
