GENERAL CHARACTERISTICS OF MINERALS. 17 



erals, there are various qualities presented for our study. 

 We observe that stones or minerals have color ; they have 

 hardness in different degrees, from being soft and impressi- 

 ble by the nail, to the extreme hardness of the diamond ; 

 they have weight ; they have luster, from almost a total ab- 

 sence of the power of reflecting light to the brilliancy of a 

 mirror. Some are as transparent as glass and others are 

 opaque. A few have tasto. These are the most obvious 

 characters, and characters to which the mind would at once 

 appeal in distinguishing species. 



Other characters of equal importance are found in the 

 internal and external structure of minerals. On examining 

 a piece of coarse granite, we find that each scale of mica 

 may be split by the point of a knife into thinner leaves. 

 Here is evidence of a peculiar structure, called cleavage ; 

 and wherever mica is found, this peculiarity is constant. 

 The feldspar in the same rock, if examined with care, will 

 be found to break in certain directions with a smooth, or 

 nearly smooth plain surface, showing a luster approaching 

 that of glass, though somewhat pearly. It is true of feldspar 

 also, that this cleavage is a constant character for the spe- 

 cies, as regards direction and facility. In nearly all miner, 

 als, this kind of structure, more or less perfect in quality, 

 may be distinguished. In a broken bar of iron the irregu- 

 larity of the grains proceeds from this cause. In granular 

 marble, although the mass as a whole has no such structure, 

 the several grains if attentively examined will be seen to 

 present a distinct cleavage structure and consequent angu- 

 lar forms. In finer varieties, the grains may be so small 

 that the characters cannot be observed ; or again the tex- 

 ture of the mass may be so compact that not even grains 

 can be distinguished. 



This cleavage, then, is a peculiarity of internal structure. 

 It is intimately connected with another fact, — that these same 

 minerals often occur under the form of some regular solid 

 with neat plane surfaces ; and are finished with a symmetry 

 and perfection which art would fail to imitate. These forms 

 are their natural forms, and every mineral has its own dis- 

 tinct system of forms. The beauty of a cabinet of min- 

 erals arises to a great extent from the variety of forms and 



What physical characters are to be observed in the stud}* of min- 

 erals? What character depends on internal structure? Mention ex- 

 ample! and explain. What other character depends on structure ? 

 2* 



