62 



OUR PHYSICAL WORLD 



KEY TO COMMON ROCK-FORMING MINERALS Continued 



Feldspar,* 6.0-6.5 Many colors, streak white. 

 Cleavage perfect, faces at nearly 

 right angles. Light colored, 

 orthoclase; darker, plagioclase. 



Pyrite, 6.0-6.5 Brass yellow, tarnishes brown. 



(Fool's gold) Streak greenish black. Metal- 



lic luster. Crystals, cubes or 

 dodecahedra. Harder than 

 chalcopyrite. 



'Olivine, 6.5-7.0 Green, streak white. Trans- 



parent to translucent. Usually 

 occurs in rounded grains. 



Quartz, 7 Color anything from black to 



As hard as white. Luster vitreous or waxy 



quartz in chalcedony. Fracture con- 



choidal. Crystals six-sided 

 prisms ending in pyramids; 

 blue, amethyst, banded agate, 

 onyx, jasper. In massive 

 nodules occurs as flint. 



* The term feldspar stands for a group of minerals. Orthoclase is a silicate of 

 aluminium and potassium a "potash-feldspar." Its cleavage angle is a right 

 angle, or nearly so. It is usually light in color, white, gray, pink. It commonly 

 occurs in rocks in which quartz is present fairly abundantly and seldom associates 

 with the plagioclase group. This plagioclase group includes the soda-lime feld- 

 spars like oligoclase and labradorite. The plagioclases have an oblique cleavage 

 angle, and certain cleavage faces are marked with numerous fine parallel lines. 

 The plagioclases, especially the oligoclase and the labradorite, -are strongly basic, 

 seldom occur with quartz in any quantity, often are present with augite or horn- 

 blendes. They are usually dark colored, blues, grays, or dull reds. 



Rocks are constantly forming nowadays. When from some 

 great volcano there is an outflow of lava, and this molten material 

 cools and solidifies, it forms rock (Fig. 29). Such rocks, formed 

 from the cooling of a molten mass, are known as igneous rocks. 

 The wear and tear of the waves, ocean currents, and other agents 

 of erosion disintegrate rocks, and the debris is carried out to sea. 

 Offshore this material is being deposited as great beds of sand 

 and mud. As this process goes on through countless years the 

 deposits thicken, and the lower strata, subject to the vast pressure 



