The latter is close to the average composition of the continental crust of the Earth. Ideally it would be preferable to use the same modal scheme for volcanic rocks. One popular scheme is based on the use of both chemical components and normative mineralogy. Because most lay people have little access to analytic facilities that yield igneous rock compositions, only an outline will be presented here in order to provide an appreciation for the classification scheme. Chemically the subalkaline rocks are saturated with respect to silica.
Plagioclase is common in phenocrysts, but it can also occur in the matrix along with the pyroxenes.
Hornblende and biotite phenocrysts are common in calc-alkaline andesites and dacites but are lacking in the tholeiites. Hornblende and plagioclase phenocrysts are more widespread in dacites than in rhyolites, which have more biotite and alkali feldspar. We call this a field classification because it requires little detailed knowledge of rocks and can be easily applied to any igneous rock we might pick up while on a field trip. It utilizes texture, mineralogy and color. Students the thus cautioned to use color only as a last resort.