Page:Transactions of the Geological Society, 1st series, vol. 1.djvu/44

32 The primitive Crystal and its varieties.



Fig. 1 is the primitive crystal; the angle formed by the meeting of the two faces PP has been ascertained by Haüy to be 109d. 28′, 16″. Fig. 2 is the result of a decrease on one face of the upper, and on the opposed face of the lower pyramid. This crystal has aptly been called the segment of the octahedron, as the explanatory fig. 3, will evince. Fig. 4 is the primitive crystal elongated; this elongation arises, as is obvious, from a regular increase of the crystalline laminæ on one face of the upper pyramid, and on that face of the lower to which it is united at their common base. Fig. 5, is the same occurring in capillary crystals. Fig. 6 differs from fig. 4 in the upper pyramid only; on the already increased face of which, a still further increase of laminæ, has taken place, as well as a similar deposition on the opposite face of the same pyramid. Fig. 7 is produced by an increase on one face of the upper, and on one face of the lower pyramid of fig. 4, as will be obvious on consulting fig. 8. Fig. 9 is the consequence of an increase on two opposed faces of the upper, and on the other two opposed faces of the lower pyramid of the primitive crystal, as will be seen by fig. 10. Fig. 11 is the result