Handbook of Paleontology 69 



order are biotite mica, hornblendes, pyroxenes and 

 olivine. Biotite and hornblendes indicate basic rocks, 

 pyroxenes greater basicity, and only the most basic 

 igneous rocks have olivine. Free quartz is usually not 

 present when there is pyroxene and olivine. Though 

 some of the hornblendes are white to gray in color, 

 they usually vary from gray-green through bright 

 greens to darker greens and black. Pyroxenes have 

 about the same colors and olivine varies from olive- 

 green to yellow-green. The beginner can not hope to 

 distinguish the hornblendes, pyroxenes and olivine, 

 but usually the lack of free quartz will be an indica- 

 tion of the more basic pyroxene and olivine. When 

 minerals crystallize from molten magma the more 

 basic minerals crystallize out first and quartz, if there 

 is any silica left over uncombined, separates out last 

 and fills the spaces between the other minerals, hence its 

 irregular form. Thus the order of crystallization in 

 igneous rocks is ores or oxides of iron (as magnetite), 

 ferromagnesian minerals (as pyroxenes, hornblendes 

 etc.), soda-lime feldspars, potash or potash-soda feld- 

 spars, quartz. The minerals first developed have more 

 regular forms, because they have more room in which 

 to crystallize. 



Granite occurs in the rock group which contains the 

 most acidic of the common igneous rocks. It is a granu- 

 lar rock, the coarse-grained forms of which are composed 

 almost entirely of orthoclase feldspar and quartz. Gen- 

 erally two kinds of feldspar are present, orthoclase and 

 acid plagioclase, and they may as a general rule be dis- 

 tinguished by their color. Often muscovite mica, biotite 

 mica and hornblende are present and recognizable by their 

 dark colors. Mica is very soft and scaly with a high 



