458 W. J. MILLER CLASSIFICATION OF METAMORPHIC ROCKS 



MerrilP^ says schists ^%re a large and extremely variable series of rocks, 

 differing from the gneisses mainly in the lack of feldspar as an essential 

 constituent." A foliated granite is, accordingly, a gneiss, while a foliated 

 peridotite is a schist. Thus no structural difference between the gneisses 

 and schists is made, and this is very unusual among American geologists. 



According to Kernp,^" schists are "thinly laminated, metamorphic rocks 

 which split more or less readily along certain planes approximately paral- 

 lel." Strictly in harmony with this statement, slates would be included 

 with schists. They would be excluded b;;^ limiting the schists to mega- 

 scopically crystalline metamorphic rocks. 



Finlay^^ says : "Schist is a foliated metamorphic rock which will split 

 into thin slabs or flakes much more readily than gneiss does." Here, 

 again, slates would be included. 



Keeping in mind the above statements regarding the meaning of the 

 term "schist," the writer proposes the following definition : A schist dif- 

 fers from a gneiss in being more finely foliated and capable of being split 

 (cleaved) into thin layers. Such a rock possesses a schistose structure. 

 As thus defined, schists are simply more or less perfectly cleavable 

 gneisses, and the gneissose structure passes into the schistose structure by 

 insensible gradations. 



Meta-igneous Rocks^^ 



foliates 



Primary ortho^^ -foliates. — These foliated igneous rocks have been pro- 

 duced essentially by magmatic flowage or differential movements in 

 igneous massesi before complete consolidation. Because their foliation 

 has been developed before final consolidation of magmas, they are called 

 "primary" foliates. These rocks comprise the so-called "primary 

 gneisses," but' the writer strongly urges that these be removed from the 

 category of true gneisses, (1) because their foliation is not strictly of 

 secondary origin, as is true of all other rocks called gneisses, and (2) 

 because they are really intermediate between truly unaltered and altered 

 igneous rocks. In fact, there is some ground for excluding these from 

 the class of metamorphic rocks altogether ; but, because ( 1 ) they possess 

 genuine foliation which is so characteristic of most of the common meta- 



20 G. p. Merrill : Rocks, rock weathering, and soils, 1906, p. 146. 



30 J. F. Kemp : Handbook of Rocks, 1911, p. 252. 



31 G. I. Finlay : Igneous rocks, 1913, p. 14. 



32 Instead of the long terms "meta-igneons rocks" and "meta-sedimentary rocks," it is 

 suggested that "meta-igs" and "meta-seds," respectively, be used, these being short, con- 

 venient, self-explanatory designations. Clearly, however, the possible use of these terms 

 would in no wise affect the principles of the classification set forth in this paper. 



33 The prefixes "ortho-" and "para-" are used in this paper in the generally accepted 

 sense to indicate metamorphosed igneous and. sedimentary rocks, respectively. 



