26 



EARTH FEATURES AND THEIR MEANING 



tered relics they are piled away together in a bed of sand. Yet 

 no amount of such rough handling is sufficient to destroy the crys- 

 tal individuality, and if they are now surrounded with conditions 

 which are suitable for their growth, their individual nature again 

 becomes revealed in new crystal outlines. Many of our sand- 

 stones when turned in the bright sunlight send out flashes of light 

 to rival a bank of snow in early spring. These bright flashes 

 proceed from the facets of minute crystals formed about each 

 rounded grain of the sand, and if we examine them under a lens, 

 we may note the beauty of line formed with such exactness that 

 the most delicate instruments can detect no difference between 

 the similar angles of neighboring crystals (Fig. 12). 



FIG. 12. Battered sand grains which have taken on a new lease of life and have 

 developed a crystal form, a, a single grain grown into an individual crystal ; b, 

 a parallel growth about a single grain ; c, growth of neighboring grains until they 

 have mutually interfered and so destroyed the crystal facets the common con- 

 dition within the mass of a rock (after Irving and Van Hise). 



This individual nature of the crystal is believed to reside in a 

 symmetrical grouping of the chemical molecules of the substance 

 into larger and so-called " crystal molecules." The crystal quality 

 belongs to the chemical elements and to their compounds in the 

 solid condition, but not to ordinary mixtures of them. 



Some properties of natural crystals, minerals. No two mineral 

 species appear in crystals of the same appearance, any more 

 than two animal species have been given the same form ; and so 

 minerals may be recognized by the individual peculiarities of their 

 crystals. Yet for the reason that crystals have so generally been 

 prevented from developing or retaining their characteristic faces, 



