CRYSTALS. 



427 



SILICATES, such as sand and flint, are in daily demand. Agate, cornelian, 

 Scotch pebbles, rock-crystal, etc., belong to the same family. Even our 

 gems are crystallized earths, and, as already stated, diamonds are merely 

 carbon. 



Stone, as we know, is quarried ; that is, it is dug out of the earth. 

 But perhaps many readers do not know why a stone-mine is called a 

 " quarry." Most kinds of stone (granite and marble are the exceptions) are 

 found in layers, or strata, rendering them easy of removal. The blocks of 

 stone are cut with reference to these layers in a more or less square manner, 

 and "squared up" before they are carried away. Thus the term "quarry," 

 from an old French word, qitci)r<,\ or earn*, as now written, signifying a 

 square. In granite quarries the stone bsing very hard is bored, and loosened 

 by means of gunpowder or dynamite blasting. Slate, on the contrary, is 

 easily divided into slabs. We will now resume? the subject of Crystals. 



J i. Emerald. 3. Garnet. 



^* ( 2. Agate. 4. Ruby. 



;. Diamond. 

 5. Rock crystal. 



We have said that crystals vary in size, and this variety may be traced, 

 in the cases of crystallization from fluids, to the slowness or the rapidity of 

 the cooling process. If the work be done slowly, then the crystals obtain a 

 size commensurate with the time of cooling, as they are deposited one upon 

 the other. The form of minerals is the first important point, and to ascer- 

 tain their forms and structure we must study CRYSTALLOGRAPHY. We 

 shall find faces, or planes, the lines of contact of any two planes, called 

 edges, and the angles formed where these planes meet. We may add that 

 crystals have, at least, four planes, making six edges and four angles. Nearly 

 all crystals have more than this, for the forms are, if not infinite, very 

 numerous, and are divided into six (by some writers into seven) different 

 systems or fundamental forms from which the varieties are derived. The axis 

 of a crystal is an imaginary line drawn from an angle to the opposite one. 



The first form, the monomctric, or cubic system, with three equal axes 



