20 THE MINEBALS CONSTITUTING ROCKS 



The most striking features of these analyses are (1) the 

 complete conversion of the protoxides into sesqiiioxides, (2) the 

 loss in lime and magnesia which have presumably been carried 

 away in the form of carbonates, and (3) the assumption of 8% 

 of water. As the dark aluminous and ferruginous hornblendes 

 are among the commonest and most wide-spread of minerals, it 

 is apparent from the above that they may have an important 

 bearing upon the color and physical qualities of the residual 

 clays; to which they thus give rise. The peroxidation of the 

 iron gives yellow, brown, or red colors, while the hydrated 

 aluminous silicate (clay) imparts tenacity. The final product 

 of such decomposition is, then, a ferruginous clay as already 

 noted. 



The Pyroxenes. — The rock-forming pyroxenes are divided 

 upon erystallographie grounds into two groups, the one ortho- 

 rhombic in crystallization, and the other monoclinic. All varie- 

 ties, when in good crystalline form, show in basal sections an 

 octagonal outline bounded by prismatic and pinacoidal faces, 

 with a well-defined cleavage parallel with the prism faces. 

 Chemically they are silicates of magnesia and iron with lime 

 and alumina in varying proportions. They are hard, tough 

 minerals and have an important bearing upon the physical 

 properties of the rocks of which they form a part. 



The Monoclinic Pyroxenes. — Two principal varieties are 

 recognized. (1) Pyroxenes containing little or no alumina, and 

 composed of silica, 45.95 to 55.6% ; lime, 21,06 to 25.9% ; mag- 

 nesia, 13.08 to 18.5%, with sometimes varying quantities of iron 

 oxides and water. Under this head are included the lighter 

 colored varieties, malacolite, sahlite, and diallage. (2) Pyroxenes 

 containing alumina, and composed of silica, 49.40 to 51.50% ; alu- 

 mina, 6,15 to 6.70% ; magnesia, 13.06 to 17.69% j lime, 21.88 

 to 23.80%; iron oxides, 0.35 to 7.83%, with sometimes small 

 quantities of soda and water. Under this head are included 

 the darker varieties, augite and leucaugite. 



The lighter colored, non-aluminous varieties, malacolite and 

 sahlite, are common in mica and hornblendie schists, gneiss, 

 and granite, though not always in sufficient abundance to be 

 noticeable to the naked eye. The foliated variety, diallage, 

 is an essential constituent of the rock gabbro, and is also 

 common in peridotites. The darker colored, aluminous vari- 

 ety, augite, is an essential constituent of diabase and basalt. 



