46 



STRUCTURE OF MINERALS. 



of inclination remains the same : and this is a general fact 

 in all crystals, that whatever distortions take place, the angles 

 are constant. Greater diversity is given to the shapes of 

 crystals by these simple variations, without multiplying the 

 number of distinct forms. Figure 83 is a tapering prism of 

 the same mineral, with a minute pyramid at the apex. The 

 faces of this pyramid have exactly the same inclinations as 

 those of figure 79. 



The constancy of the angles shows that the fundamental 

 form of the crystal, or, in other words, the form of its mole- 

 cules, is constant, amid alL these variations of size and shape. 



Crystals have sometimes curved faces. The faces of 

 diamonds are usually convex, and some crystals are almost 



84 spheres. Figure 84 is one of these diamond 

 crystals. It is the same form as is represented 

 in figure 45. For cutting glass, they always 

 select those crystals that have a natural curved 

 edge, as others are much inferior for the purpose 

 and sooner wear out. In figure 85 a different 



kind of curvature is represented. It is a curved rhombohe- 



85 dron, in which the opposite faces are parallel in 

 their curving : it is a common form of spathic iron 

 >and pearl spar. The latter mineral from Lock- 

 port, New York, is always curved in this way. 



Si ill more singular curvatures are sometimes 

 met with. In the mammoth cave of Kentucky, 

 leaves, vines and flowers are beautifully imita- 

 ted in alabaster. Some of the " rosettes" are 

 a foot in diameter, and consist of curving leaves, 

 clustered in graceful shapes. The frostings on 

 our windows in winter are often miniature pic- 

 tures of forests and vines with rolled tendrils. 

 'It is one among the many singular results of 

 ^crystallization. On the cool mornings of spring 

 or autumn, in this climate, twigs of plants are 

 occasionally found 'encircled by fibrous icy 

 curls, (fig. 86,) which are attached vertically to 

 the stem. They are formed during the night, 

 and disappear soon after the appearance of the 

 sun. 



What is said of curved crystals 1 

 gypsum 1 of ice ? 



What of curved crystallizations of 



