58 PHYSIOGRAPHY. [chap. 



assumed forms which are extremely definite in themselves, 

 and are known as crystals. 



In the rocks of Snowdon, and of many other parts of 

 Britain, there may be found a beautifully transparent sub- 

 stance, of great hardness, which puts on very definite 

 shapes. These shapes, as represented in Fig. 15, usually 

 look like little six-sided towers, shooting in all directions 

 from the rock, and terminated at one end, or sometimes 

 at each end, by a short six-sided spire. The faces are as 

 smooth and bright as though they had been just polished 



Fig. 15. — Rock crystal. ' 



on the lapidary's wheel ; whilst the edges are as sharp and 

 straight as though cut by a skilful workman. The ancients, 

 who were familiar with these clear and colourless solid 

 bodies as they occur in the granitic rocks of the Alps, 

 supposed that they were composed of ice; that they 



^ The lines on these crystals indicate shading, and not markings on 

 the natural specimen. The prisms of rock-crystal are often marked by- 

 lines, but they run across the prisms, not longitudinally in the direction 

 of the shading in the figure. 



