THE EARTH'S ROCK FOUNDATIONS 65 



feldspars crystallize out before the orthoclase, and quartz seems 

 to be one of the last to crystallize. Not infrequently one finds 

 a rock composed of a finely crystalline ground-mass containing 

 large and distinct crystals of some constituent mineral. Such 

 rocks are designated porphyries. In the porous rocks the cavi- 

 ties formed by gas bubbles have in some cases later been filled 

 with some mineral deposited usually by water percolating through 

 the rock. Such rocks with more or less spherical masses of 

 mineral deposited in the cavities are known as amygdaloids. 



Not only do the igneous rocks differ in texture but they differ 

 also in chemical composition according to the prevalence of 

 the various minerals. As noted already, most of the important 

 minerals entering into the formation of igneous rocks are silicates. 

 When metals combine with silica some of them take up large 

 quantities of silica, others relatively small quantities. This 

 depends upon the valence of the metal. Thus iron has a valence 

 of four, as does manganese; while sodium and potassium have a 

 valence of only one; calcium a valence of two. This means 

 that iron is capable of combining with four atoms of monovalent 

 substances, like hydrogen, say, while sodium can only combine 

 with one. When, therefore, such a metal is combining with silica 

 to form a silicate, the element with the greater valence will take 

 up much more of the silica. The silicates of such metals as 

 sodium and potassium, as we have seen in the orthoclase feld- 

 spars, are likely to be light in color and light in weight as com- 

 pared with the minerals that are silicates of the heavy metals like 

 iron and manganese, such as pyroxene and hornblende. The 

 rocks formed from the combination of such light-colored and 

 lightweight minerals are also prone to contain a great deal of 

 free silica in the form of quartz, whereas, for the reason just 

 given, the silica is not likely to be free in rocks made of the 

 darker and heavier metals. 



On the basis of these two characters the texture of the 

 mineral and the prevalence of certain constituent minerals we 

 can classify the rocks. In the accompanying tabulation (p. 66) , the 



