200 IGNEOUS ROCKS 



III. The Diorite Family 



The magma of these rocks has about the same silica percent- 

 ages (55 to 65 %) as have the syenites, but the quantity of alkalies 

 is less, while that of the lime and magnesia is greater. Hence 

 orthoclase is absent or much less important, and the principal 

 mineral is a soda-lime felspar. The textures display the usual 

 variety from glassy to granitoid. 



The glasses of this family (andesite obsidian) can be distin- 

 guished from those of the preceding groups only by chemical 

 analysis, but they are rare. 



Andesites are dark-coloured lavas of porphyritic or compact 

 texture, composed of a glassy plagioclase felspar and some ferro- 

 magnesian mineral, embedded in a ground mass of felspar needles 

 and glass. In accordance with the nature of the predominant 

 ferro-magnesian mineral, we have hornblende andesite, biotite ande- 

 site, and several varieties of pyroxene andesite. These rocks are 

 very common in the western United States and along the Pacific 

 coast of both North and South America ; they are named from 

 the Andes. 



The Dacites differ from the andesites in having quartz, and 

 therefore a higher percentage of silica. 



The Diorites are the plutonic equivalents of the andesites and 

 dacites, having granitoid texture, but they are usually of much 

 finer grain than the granites and syenites. The ferro-magnesian 

 mineral is usually green hornblende, but augite and other pyrox- 

 enes and biotite occur in the different varieties. Most diorites 

 have a little quartz ; but when this mineral becomes abundant, it 

 gives a quartz diorite, which is related to the dacites as the typical 

 diorite is to the andesites. A common name for the diorites is 

 greenstone. 



IV. The Basalt Family 



In the magmas of this series the percentage of silica is much 

 less than in the preceding groups (40 to 55 %), and the quantity 



