Division of igneous rock s on the basis of their silicate mineral content, these minerals usually being the most abundant in such rocks.
Rocks are described as acid, intermediate, basic, and ultrabasic, in order of decreasing silica content, because it was earlier thought that silica is present in rock magma s in the form of silicic acid. In modern usage the terms do not refer to acidity in the chemical sense. In general, the gradation from acid to basic corresponds to an increase in colour (i.e., light to dark).