318 ON A NEW STRUCTURE IN APOPHYLLITE, AND 



at the angles very minute truncations, inclined 120° to the 

 edges of the prism *, or 150° to the summit. The sides of the 

 rectangles are nearly equal, and are commonly about y^th 

 of an inch, while the length of the prism seldom exceeds 

 2|th tenths of an inch. The four faces of the prism have 

 an irregular surface, with longitudinal striae, but are high- 

 ly polished ; while the flat summits display an inferior lustre, 

 and are variegated with the pearly tints which have given to 

 this mineral one of its most common names f, and which arise 

 from the imperfect contact of the elementary laminae. These 

 laminae, whose surfaces are perpendicular to the axis of the 

 prism, may be easily separated, to any degree of thinness, by 

 applying, with the hand, the edge of a sharp knife or lancet ; 

 and it is no doubt owing to the laminated structure, and to the 

 imperfect surfaces of the laminae, that a dilute acid, which will 

 not corrode the polished faces of the prism, will act freely on 

 its less resisting summits. 



When we remove the uppermost slice from each of the two 

 summits of the crystal, to the thickness of the 100th of an 

 inch or more, and examine it either by the microscope or by 

 polarised light, we perceive no tesselated structure. A num- 

 ber of veins appear at the edges MO, ON, NP, PM, as shewn 

 in Plate XX. Fig. 1., or at A, Fig. 2. and these veins gradu- 

 ally diminish in number the nearer we come to the summit, 

 though they never disappear. 



If we now remove the next slice, and all subsequent 

 slices, between" the two summits, we shall find that they exhi- 

 bit 



* Hauy makes this angle 117° 48'. 



-f- Ichthyophthalmite, or Fish-Eye Stone. These pearly tints occur only in one 

 variety of the Tesselite. 



