22 MINERALOGY. 



Fig. 1 7, the particles or molecules being 

 in reality so minute that no inequality 

 is perceptible to the eye on the surface 

 of the crystal. 



27. Constancy in the Forms of Crys- 

 tals. Notwithstanding the immense 

 Fig. 17. variety in the secondary forms of crys- 



tals, there is, in the case of each mineral, a strict adher- 

 ence to its own form or forms in all essential character- 

 istics. The size and number of faces may differ in dif- 

 ferent specimens so much as to give them an entirely 

 different appearance, but the mineralogist will discover 

 their relation to each other by observing that the corre- 

 sponding angles are exactly the same, and by cleavage 

 he will find the same fundamental form. " Crystals are, 

 therefore," says Professor Dana, "the perfect individuals 

 of the mineral kingdom. The mineral quartz has a spe- 

 cific form and structure, as much as a dog or an elm, 

 and is as distinct and unvarying as regards essential char- 

 acters, although, owing to counteracting causes during 

 formation, these for-ms are not always assumed. In 

 whatever part of the world crystals of quartz may be 

 obtained, they are fundamentally identical." 



28. Symmetry. In all the modifications of crystals 

 seen in the production of secondary forms there is a 

 wonderful symmetry. If one part is modified, all the 

 corresponding parts are modified, and in the same way, 

 either over the whole crystal or over just one half of it. 

 Thus, if we find a plane in place of an edge, we shall find 

 the other edges also replaced by planes of precisely the 

 same width, as seen in Fig. 8 ; and so of the angles 

 also, as represented in Fig. 7. So, if there be two planes 

 in place of any edge or angle, there will be found the 

 same planes replacing the other edges or angles ; and 

 the same is true if there be many replacing planes, as 

 there often are. You see how this secures a regularity 

 pleasing to the eye in the midst of extreme variety, thus 



