THE GLACIAL PERIOD 



73 



Feldspar,* 6.0-6.5 



Pyrite, 6.0-6.5 

 (Fool's gold) 



Olivine, 6.5-7.0 

 Quartz, 7 



Many colors, streak white. 

 Cleavage perfect, faces at nearly 

 right angles. Light colored, 

 orthoclase; darker, plagioclase. 



Brass yellow, tarnishes brown. 

 Streak greenish black. Metal- 

 lic luster. Crystals, cubes or 

 dodecahedra . Harder than 

 chalcopyrite. 



Green, streak white. Trans- 

 parent to translucent. Usually 

 occurs in rounded grains. 



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 

 aluminum 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. 



Igneous rocks. Sedimentary rocks are largely formed from 

 the disintegration of previously existing rocks. The original 

 rock masses, together with many of those of later times, were 

 formed from the cooling of molten material. Such are igneous 

 rocks. When lava outpours on the earth's surface in volcanic 

 regions it forms rock as it cools volcanic rock. Such rock 

 usually has a glassy appearance or, if it is crystalline, the crystals 

 of which it is composed are small, usually indistinguishable, for 

 it cools too rapidly to permit a thorough crystallization. Such 

 volcanic rocks are liable to be quite porous on account of the 



