776 A TREATISE ON METAMORPHISM. 



a classification of such rocks, but there is not even an approximation to a 

 consensus of opinion as to the principles upon which a classification should 

 be based. Until the original igneous rocks are satisfactorily classified it is 

 quite impossible satisfactorily to consider their metamorphosed equivalents. 

 The situation in reference to the original sedimentary rocks is somewhat 

 more fortunate. There is rather general agreement as to the principles 

 upon which the classification of the sedimentary rocks is to be based, and 

 upon the main classes, orders, aud families of rocks which shall be 

 recognized. I shall therefore attempt to consider the metamorphism of 

 each of the main divisions of the altered sedimentary rocks, but shall not 

 make a similar attempt for the igneous rocks. 



USE OF SOME GEjSTERAE TERMS APPLIED TO METAMORPHIC ROCKS. 



The more important of the terms generally applied to metamorphic 

 rocks are the prefixes meta and apo, and the general names "slate," "schist," 

 and "gneiss." 



META. 



The prefix meta is used in this treatise in a general way to indicate 

 any kind of alteration of any kind of rock. This usage of the prefix 

 corresponds with the definition of the term "metamorphism" as given in 

 Chapter I, page 32, where metamorphism is defined as meaning all kinds 

 of alterations of all rocks by all forces, agents, and processes. In the sense 

 proposed we may say meta-sandstone, meta-shale, meta-arkose, meta- 

 dolerite, etc. We may even apply the prefix meta to a rock already 

 metamorphosed which has undergone a second set of changes, as, for 

 instance, meta-graywacke and meta-quartzite. Thus used it means that a 

 rock which had first been transformed to a graywacke or quartzite, respec- 

 tively, was afterwards again metamorphosed. 



The prefix apo is here used as a qualifier to indicate metasomatic 

 cnanges in rocks in which the original textures and structures are largely 

 preserved. This usage of apo accords with the underlying idea of the term 

 as proposed by Doctor Bascoin." She proposed to call a rhyolite which has 



"Bascom, Florence, Aporhyolite of South Mountain, Pennsylvania: Bull. Geol. Soc. America, 

 vol. 8, 1897, pp. 393-396. 



