14 MINERALOGY. 



Granite is in contrast with marble in this respect. While 

 the latter is composed of one compound mineral, the 

 carbonate of lime, the former has mingled together in a 

 confused manner three compounds mica, feldspar, and 

 quartz. The eye can generally distinguish readily the 

 three kinds of crystals imperfectly formed. A full con- 

 sideration of the composition of the rocks will claim our 

 attention in another chapter. 



12. Mineralogy, Geology, and Chemistry. While Min- 

 eralogy has an intimate relation with Chemistry, it dif- 

 fers from it. It gets from Chemistry the composition of 

 the various minerals of which it treats, but it has noth- 

 ing to do, as Chemistry has, with what can be made out 

 of these substances, or what can be extracted from them. 

 It treats of substances simply as they are found in na- 

 ture, without regard to any action that might be induced 

 by mingling them together. Mineralogy is preparatory 

 to Geology, because it gives us a knowledge of the min- 

 eral substances which make up the rocks. The three 

 sciences are thus, as you see, linked together, and the 

 proper order for their study is that which I have adopted. 



CHAPTER II. 



CONSTRUCTION OF MINERALS. 



13. Crystallization. When a mineral substance takes 

 on a solid form, the atoms or particles are disposed to an 

 arrangement which is termed crystallization. It is a 

 very definite and exact arrangement, with straight lines, 

 perfect angles, and plane, smooth faces. Crystals of 

 quartz, commonly called rock crystal, are familiar exam- 

 ples. Mica is another of quite a different kind, the crys- 

 tals being foliated that is, in leaves. When the proc- 

 ess of forming the crystal is not interfered with by any 

 circumstance, as, for example, agitation, all parts of it 



