THE EARTH'S ROCK FOUNDATIONS 59 



and are six-sided in cross-section, the faces meeting at angles 

 like those made by the cleavage faces. The luster on freshly 

 broken surfaces is bright and vitreous, while in pyroxene it is 

 commonly dull. Hornblende occurs sometimes in a finely 

 columnar or even fibrous form known as asbestos; then the 

 luster is silky. 



Olivine is an olive-green to bottle-green mineral, harder than 

 pyroxene or hornblende (6.5-7). It is transparent to trans- 

 lucent. It cleaves only in one direction. It occurs in the igne- 

 ous rocks in grains, and might be mistaken for the preceding 

 minerals at first sight, but its greater hardness and cleavage in 

 only one direction will distinguish it. 



The micas are readily distinguished because they cleave so 

 readily into very thin elastic plates. The commonest ones in 

 rocks are muscovite, a light-colored one, which is a hydrated 

 silicate of potassium and aluminium; and biotite, dark brown 

 to black, a hydrated silicate of iron, magnesium, and aluminium. 



Kaolin, which is a very pure clay, results from the disin- 

 tegration of the feldspars or similar minerals in the presence of 

 water and carbon dioxide. It is a silicate of aluminium com- 

 bined with water (H 4 Al 2 Si 2 O 9 ) . It usually occurs in great masses 

 or beds, is soft, white, and has a greasy feel when rubbed 

 between the fingers. It is readily tinged with impurities, becom- 

 ing yellow, brown, or gray. It also occurs in beds more or less 

 mixed with other substances sand, mica, hematite, organic 

 matter, etc. and so gives the ordinary clays. Such beds are 

 important in rock formation, for out of them have been made 

 some important sedimentary and metamorphic rocks. 



Chlorite is another hydrous silicate resulting from the weather- 

 ing of the anhydrous sorts. In reality there are several chlorites, 

 but all are much alike and may be treated here under the one 

 heading. The color is green; the cleavage is much like that of 

 mica, but the flakes, while bending easily, are inelastic and remain 

 bent instead of springing back to their original form as do the 

 micas. Chlorite is so soft, too, that it is scratched by the finger 



