This section is from the book "An Introduction To Geology", by William B. Scott. Also available from Amazon: An Introduction to Geology.
The magma of these rocks has about the same silica percentages (50 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 distinguished 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 andesite, 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. The ferro-magnesian mineral is usually green hornblende, but augite and other pyroxenes 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.