Review of Dana's Mineralogy. 



365 



Fig. 2. 



ry or primary planes of the crystals, 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 

 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 strise on prismatic crystals of quartz, 

 (Fig. 1,) are examples of this combination, in which 

 the oscillation has taken place between the prismatic 

 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 

 the prism. In this manner, crystals of quartz are often tapered to a 

 point, without the usual pyramidal terminations." 



" Diagonal strise 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 ; 

 the former resulting from an oscillatory combina- 

 tion of the dodecahedron with the regular octa- 

 hedron, and the latter, 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- 

 nets are often striated parallel with the symmet- 

 rical 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 

 interfacial 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." 



