Introduction The purpose of this paper is to present a comprehensive classification of metamorphic rocks, the attempt being made to work out a satisfactory scheme without the introduction of new terms. In this proposed classification the main principles are based on (1) origin of the rocks, (2) rock structure, and (3) composition. Since it is necessary to have certain important terms pertaining to metamorphic rocks clearly, defined and differentiated, this matter is considered at some length. After the terms are defined, they are, it is believed, consistently employed in the classification presented. In geological literature there is much confusion regarding the nomenclature of metamorphic rocks, and, in many eases, after a writer has clearly defined a term this same term is used in a different sense in the same book or paper. Apparently there are few published attempts to make comprehensive classifications of metamorphic rocks. Many text-books, as, for example, Pirsson’s . . .