INTRODUCTION 453 



plagioclase gneisses, (4) eclogite and amphibolite, (5) magnesian-silicate 

 gneisses, (6) jadeite rocks, (7) chloro-melanite rocks, (8) quartzitic 

 rocks, (9) lime-silicate rocks, (10) marbles, (11) iron oxide rocks, and 

 (12) alumina-oxide rocks. Further, the rocks in each group are regarded 

 as having been produced in three zones within the earth^s crust, namely, 

 an upper, a middle, and a lower. It is argued that each of these zones is 

 dominated by certain definite physical conditions, resulting in the pro- 

 duction of rock types characteristic of each zone. It is the present 

 writer^s belief that this classification is altogether too elaborate and arti- 

 ficial to be practicable in the light of our present knowledge of these rocks. 

 In the first place, twelve really well defined groups of schists and gneisses, 

 based on chemical composition, can not be made, because there are endless 

 gradations back and forth from one group into another. In the second 

 place, the attempt to classify all schists and gneisses into three zones 

 according to depth reminds us of the idea, long maintained by Eosen- 

 busch, that igneous rocks show characteristics whereby they can be classi- 

 fied as of surface, dike, or plutonic origin. In an excellent review of 

 Grubenmann's scheme, Leith and Mead^ say: "It puts an emphasis on 

 depth as a controlling factor in anamorphism which does not seem to the 

 writers to correspond to actual field observations. . . . Depth is only 

 one of the factors determining- these differences in intensity. . . . 

 Field conditions indicate that the same conditions which will produce 

 certain schists of the ^upper^ group from one type of rock may produce 

 schists of the 'middle^ or ^lower' groups from another type of rock.^^ 



Discussion of important Terms 

 metam0rfhi8m 



According to Van Hise,^ "metamorphism means any change in the con- 

 stitution of any kind of rock.'^ This is a use of the term in the very 

 broadest sense, and would include even such changes as the transformation 

 of clay into shale, mere mechanical breaking up or disintegration of rocks, 

 etcetera. Usually, however, rocks produced by the processes just men- 

 tioned are not considered to be metamorphic rocks. 



The following definition of metamorphism, based essentially on that 

 by Pirsson,'^ gives the meaning of the term as it is employed iu this paper. 

 Metamorphism means any change in mineral composition, structure, or 

 texture of an igneous or a sedimentary rock whereby the original rock 

 character has been notably changed. Though it is impossible always to 

 sharply distinguish between the original rocks and their metamorphic 



^ Leith and Mead : IMetamorphic geology, 1915, pp. 188-193. 

 G C. R. Van Hise : U. S. Geol. Survey Men. 47, 1904, p. 32. 



