THE NATIONAL INSTITUTION. 199 



Made or chiastolite is described as being always crystallized in nearly rectan- 

 gular four-sided prisms. When, however, the end of the prism is carefully ex- 

 amined, there will be perceived in it a blackish prism enclosed in the larger one, 

 which is of a greyish, yellowish, or reddish white color. From each angle of the 

 central prism a blackish line passes to the corresponding angle of the outer prism ; 

 and in each of these external angles there is commonly a small prismatic space, 

 filled with the same black matter as that of which the central prism is composed, 

 which is a dark clay Blate, the same as the rock in which the crystal is im- 

 bedded. 



As was before observed, Cleveland's description agrees with this, though, in con- 

 junction with a figure corresponding to this description, he gives, as another variety, 

 I think, a figure of a crystal with a rhombic base, the marking of which agrees 

 very well with some of my specimens. I have never found any of these crystals 

 rectangular, and but few approaching to that shape. 



In the most perfect specimens, the whole rhomboidal ciystal seems to be en- 

 veloped in a uniform coat of black, slaty matter, of a very fine grain, and easUy 

 admitting of considerable polish. When a cross section is made, (and it usually 

 breaks in these specimens pretty smoothly,) and the surface ground down and 

 polished, it will be seen that this thin black casing encloses a substance of a reddish 

 yellow color, of glistening and somewhat foliaceous or sparry appearance, some- 

 times opaque, sometimes shghtly translucent. This yellowish substance encloses a 

 smaller hollow prism, of the same blackish matter, marked merely by a thin black 

 line, and within it is contained the same reddish yellow matter, as filled tho space 

 between it and the outer coating ; giving, in fact, the appearance of a small pris- 

 matic crystal, of yellowish matter, with a blackish coat inserted in the centre of a 

 larger crystal, of the same color, while small black lines pass from the angles of the 

 inner case of black matter, to the cor.ssponding angles of the outer. These black 

 lines are very feint and delicate in the transvsrse diagonal of the crystal ; thicker 

 and more strongly marked in the longitudinal diameter ; spreading a little as they 

 approach the outer casing, so as to give the idea of being formed by a duphcature 

 of that investment. In one or two crystals, indeed, this formation is distinctly 

 marked. 



In other crystals, less perfect, the bases are more generally rhomboids than 

 rhombs. The outer black case is thicker, the yellowish contained matter less regu- 

 lar in its figure, (the angles being rounded off,) and, instead of an inner crystal re- 

 sembling the outer, it is divided, by transverse diagonals of the black matter, into 

 four portions. In some specimens these are very small, appearing on the section 

 merely like dots, whUe the bulk of the crystal is composed of the blackish coat, 

 which in these is rather grey than black. In other instances there is no regular 

 figure to the contained matter, but it is dispersed through a black crystal, in irregu. 

 lar patches, giving to the section a mottled appearance. 



Although the disposition of the separate parts differs from the description, I have 

 looked upon these two varieties as made, but of a different variety from that com. 

 monly described. Yot it is among the second variety that I have found very good 

 specimens of the intersection of crystals forming a cross. 



In a tliird variety, there is no yellowish contained matter to be distinguished. 

 Tho crystals consist wholly of blackish or grayish matter, _6ometimos with and 



