CONSTRUCTION OF MINERALS. 



19 



cumstances. This is exemplified in the huddling togeth- 

 er of imperfectly formed crystals in 

 marble, loaf sugar, etc., and in the 

 extremely varied forms of frost-work 

 and other similar crystallizations ( 

 16). Some crystals are arranged in 

 the form of a cross, as seen in Fig. 5. 

 Sometimes two similar crystals are 

 united together, and then we are said 

 to have a twin crystal. The crys- 



Fijr. 5. 



Fig. 6. 



tals of common salt are apt to as- 

 sume the hopper arrangement rep- 

 resented in Fig. 6. This is because 

 that, as the evaporation takes place, 

 the gravity of the salt makes the 

 mass sink constantly a little below 

 the surface, and each set of crystals 

 is deposited on the upper and outer edge of the preced- 

 ing set. 



23. Regularity of Form in Rocks. We have some- 

 thing akin to crystallization in a rude way in the gen- 

 eral lines and faces of rocks. We have the laminated 

 arrangement in the slate rocks, and the magnificent co- 

 lumnar arrangement in the trap rocks, as exemplified in 

 the Giant's Causeway. Then there are joints, so called, 

 running across strata or layers of rock, and dividing 

 them sometimes as evenly as if it were done with a 

 knife. All this will be fully illustrated in the geological 

 portion of this book. 



24. Cleavage. Some minerals can have layers chip- 

 ped or cleaved off, leaving as smooth surfaces as before. 



is n, very familiar example of cleavage from planes. 

 Sometimes the cleavage can 

 be made from angles, as 

 seen in Fig. V, and some- 

 times from edges, as seen in 

 Fig. 8. The planes made 

 Fig. T ng.a by cleaving a crystal are 



