THE GLACIAL PERIOD 



73 



As hard as 

 quartz 



Olivine, 6.5-7.0 



Quartz, 7 



\ 



Green, streak white. Trans- 

 parent to translucent. Usually 

 occurs in rounded grains. 



Color anything from black to 

 white. Luster vitreous or waxy 

 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-Ume feld- 

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

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

 The plagioclases, especially the ohgoclase 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 cooKng 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 

 bubbles of gas contained in the molten lava. The holes formed 

 by the included bubbles of gas may later be filled by material 

 deposited by percolating water. The grains of such deposited 

 minerals are naturally rounded, and the rock so altered is known 

 as an amygdaloid. Molten material, on the other hand, that 

 cools off slowly way down below the surface makes coarse-grained 

 rock, the crystals being large. Such rocks are called plutonic, 

 in distinction to the volcanic. If one mineral is present in large 



