The TAS classification can be used to assign names to many common types of volcanic rocks based upon the relationships between the combined alkali content and the silica content. These chemical parameters are useful, because the relative proportions of alkalis and silica play an important role in determining actual mineralogy and normative mineralogy. The classification appears to be and can be simple to use for rocks that have been chemically analyzed. Except for the following quotation from Johannsen (1937), this entry is based upon Le Maitre and others (2002).