; 
' 
3 
e: 
fe 
_the oscillation has taken place between the prismatic 
Review of Dana's Mineralogy. 365 
ry or primary planes of the cafetals; and we may suppose these ridges 
to have been formed by a continued oscillation in the operation of the 
causes that give rise, when acting uninterruptedly, 
to enlarged planes. By this means the surfaces of Fig. 1. 
a crystal are marked in parallel lines meeting at an 
angle, and constituting the ridges referred to. This 
combination of different planes in the formation of a 
‘surface has been termed the oscillatory combination. 
~The horizontal strie on prismatic crystals of quartz, 
(Fig. 1,) are examples of this combination, in which 
and pyramidal planes. As the crystals lengthened, =, 
there was apparently a continual effort to assume the 
terminal pyramidal planes, which effort was inter- 
ruptedly overcome by a strong tendency to an increase in the length of 
he prism. In this manner, crystals of: quartz are often tapered toa 
point, without the usual pyramidal terminations.” 
“ Diagonal stris sometimes occur on the faces of a cube showing an 
_ oscillatory combination between the cube and octahedron. The rhombic 
dodecahedron is often striated parallel either with 
the longer or the shorter diagonal of its faces; Fig. 2. 
the former resulting from an oscillatory combina- 
tion of the dodecahedron with the regular octa- 
hedron, and the datter, with the cube or planes 
bevelling the edges of the cube, as in Aplome. 
The accompanying figure represents a distorted 
crystal of magnetic iron from Haddam, Ct., illus- 
trating the oscillation between the octahedron and 
dodecahedron. ‘The faces of trapezohedral gar- 
hets are often striated parallel with the symmet- 
tical diagonal, showing an oscillation with the dodecahedron.” 
2. Variations in the forms and dimensions of Crystals.—‘* The sim- 
plest modification of form in crystals, consists in a simple variation in 
length or breadth, without a disparity in similar secondary planes. The 
distortion, however, extends very generally to the secondary planes, 
especially when the elongation of a crystal takes place in the direction 
of a diagonal, instead of the crystallographic axes. In many instances, 
one or more secondary planes are obliterated by the enlargement of 
others, proving a source of much perplexity to the young student. The 
<p angles remain constant, unaffected by any of these variations 
in form. 
“* As most of the difficulties in the study of crystals arise from these 
distortions, this subject is one of great importance to the student.” 
