274» DESCRIPTION OF THE APOPHYLLITE. 



Crystalline The specimeus of apophyllite I have examined exhibit a 



forms. ^g^ crystalline forms, among which the most simple is that 



seen on a groupe in the museum of natural history. It is 

 the primitive parallelopipedon, the eight solid angles of 

 which are cut off by triangular facets, o, o, o, fig. 4, The 

 angle of incidence between o and M is IJO' 50'. 



Another variety, which I call supeicorapounded, is that 

 represented fig. 5. The following are its principal angles 

 of incidence. Between M and T, go"' : M and s 121° 57' : 

 M and r, 149^: M and k, 118** 11': M and n, 135" 32': 

 M and 0, 110° 50' : M and /, 109° 32' : M and a:,^l 19° 1'. 

 Very uncom- ^he specimen, from which I determined this variety, is 

 monciystal, * one of the most remarkable, that has come under my 

 notice, since I began to study crystallography. It adhered 

 in a single poiht only, as I may say, to its support, from 

 which I separated it by a slight stroke. It followed from 

 this position, that the crystal has its terminations on every 

 side, which is itself not very common. But a still more 

 uncommon circumstance is the contrast presented by all 

 the. parts similarly situate, when we compare them with 

 each other. In general, when a crystal deviates from sym- 

 metry, it is only by the absence of a small number of 

 facets, among those that are necessary to the integrity of the 

 whole ; so that these facets appear to have escaped the laws 

 , tending to produce them merely by accident, and the 

 observer has little trouble to restore them in imagination. 

 But ia this crystal* which is represented exactly as it was 

 formed fig. 6*, there is only one of the faces situate on one 

 !side, that has a corresponding face opposite to it : none of the 

 other faces are repeated on the corresponding parts; and 

 «»'ach is the progress of the decrements, tliat several of the 

 , faces which are single, as o, riy k, &c., ought to show 

 themselves in eight different places, to leave no deficiency 

 in the form of the crystal. It required time and study, to 

 supply all these unexpressed ^'ircumstapccs of the crystaU 

 lization, and reduce this sort of sketch, composed of ten 

 faces seemingly without any connexion, to the real type of 

 the form, which exhibits a well arranged assortment of 



<? The f»c€s T', o', r', »', k'^ belong to ths back part of tbe crystal, 



forty- 



