452 A^^ J. miller — classification of metamoephtc rocks 



classification presented. In geological literature there is much confusion 

 regarding the nomenclature of metamorphic rocks, and, in many cases, 

 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 ^'Rocks and Rock Minerals," give simple groupings for con- 

 venience of discussion. In Van Hise^s great work, "A Treatise on Meta- 

 morphism," there is no comprehensive classification of metamorphic 

 rocks. In fact, Van Hise^ there says : ^^At the present time there is not 

 only no consensus of opinion concerning a classification of such rocks, but 

 there is not even an approximation to a consensus of opinion as to the 

 principles on which a classification should be based." Leith and Mead, 

 in their recent book on "Metamorphic Geology," give no classification of 

 metamorphic rocks. 



Kemp,^ in his "Handbook of Rocks," gives a fairly comprehensive 

 classification, which, in many respects, is an excellent one. The major 

 divisions are products of (1) contact metamorphism, (2) regional meta- 

 morphism, and (3) weathering. The contact rocks are subdivided into 

 (a) internal and (h) external products, while the regional rocks are sub- 

 divided into (a) gneisses derived from igneous rocks, as, for example, 

 granite gneiss, syenite gneiss, etcetera; (&) gneisses derived from sedi- 

 mentary rocks, as, for example, granitic gneiss, syenitic gneiss, etcetera; 

 (c) crystalline schists; (d) quartzites and slates; (e) crystalline lime- 

 stones, and (/) ophicalcites, serpentines, and soapstones. Some criti- 

 cisms of this scheme are as follows : The impossibility of always separating 

 the crj^stalline schists from the gneisses on the basis of differences in 

 mineralogical composition, it being required by Kemp that the gneisses 

 must have the mineralogy of the granitoid igneous rocks; the inadvisa- 

 bility of using such terms as "granitic gneiss," "syenitic gneiss," etcetera, 

 for derivatives from sedimentary rocks, this certainly not being in har- 

 mony with the excellent suggestions made by Gordon^ many years ago ; 

 and the failure to provide places for such important types as the so-called 

 primary gneisses, the injection gneisses, and gneisses and schists of un- 

 known origin. 



Perhaps the most elaborate classification of foliated rocks yet proposed 

 is that by Grubenmann.* He makes twelve groups of schists and gneisses 

 as follows: (1) orthoclase gneisses, (2) alumina-silicate gneisses, (3) 



1 C. R. Van Hise : U. S. Geol. Survey Mon. 47, 1904, p. 775. 



2 J. F. Kemp: Handbook of Rocks. 1911, p. 160. 



3 C. H. Gordon : Bull. Geol. Soc. Am., vol. 7, p. 122. 



* U. Grubenmann : Die kristallinen Schlefer, vol. 2, 1907, pp. 172-173. 



