14 MINERALS AND GEOLOGY 



of small grains or granular masses are said to have a granular struc- 

 ture ; ex. granular or saccharoidal limestone, granular gypsum, &c. 

 Finally, when the component particles are not apparent, the mineral 

 is said to have a compact structure, as in the native malleable metals, 

 obsidian, and most varieties of quartz. Hard and vitreous minerals 

 of a compact structure (ex. obsidian) generally show, when broken, a 

 conchoidal fracture, or a series of circular markings resembling the 

 lines of growth on the external surface of a bivalve shell. 



Almost all minerals, especially those of a lamellar structure, break 

 or separate more readily in certain directions than in others. This 

 peculiarity is called cleavage. When cleavage takes place in more 

 than one direction, the resulting fragments have often a perfectly 

 regular or definite form. Thus the purer specimens r of calc-spar, no 

 matter what their external form, break very readily into rhombo- 

 heclrons, which measure 105 5' over their obtuse edges. Galena, the 

 common ore of lead, yields rectangular or cubical cleavage forms j 

 whilst the cubes of fluor-spar break off most readily at the corners or 

 angles, and yield regular octahedrons (Figs. 6 and 3). 



Hardness. The hardness of a mineral is its relative power of 

 resisting abrasion, not that of resisting blows, as many of the hardest 

 minerals are exceedingly brittle. Practically, the character is of 

 great importance. By its aid, gypsum may be distinguished in a 

 moment from calc-spar or ordinary limestone, calc-spar from feldspar, 

 and copper pyrites from iron pyrites, not to mention other examples.* 

 The degree of hardness in minerals is conventionally assumed to 

 vary from 1 to 10 (1 being the lowest), as in the following scale, 

 devised by the German mineralogist, Mohs, and now generally 

 adopted : 



1. Foliated TALC. 



2. ROCK SALT, a transparent cleavable variety. 

 3 CALCAREOUS SPAR, a transparent variety. 



4. FLUOR SPAR. 



5. APATITE. 



* Gypsum may be scratched by the finger-nail ; Calc-spar and copper pyrites are scratched 

 easily by a knife ; whilst feldspar and iron pyrites are hard enough to scratch window-glass. 

 Some years ago, as mentioned by Sir William Logan, a farmer in the Ottawa district was put to 

 much expense and annoyance by mistaking feldspar for crystalline limestone, and attempting 

 to burn it into lime. On a late visit to the township of Marmora, we found, near a deserted 

 kiln, a large heap of quartz fragments, on which n similar attempt had evidently been made. 



