488 WHITMAN CROSS 



derived from sediments it is incorrect to place them in the strati- 

 fied class. In the necessity for placing coals and other carbon- 

 aceous rocks in an appe?idix is further evidence that the arrange- 

 ment under discussion is inadequate. 



The Crystalline Schists are defined by Rosenbusch as alteration 

 products of eruptive or sedimentary rocks. Both dynamic and 

 contact metamorphism are recognized as effective in producing 

 them. Rosenbusch further asserts that the changes have been 

 entirely structural and molecular, and not chemical, hence by 

 quantitative analysis of a metamorphic or crystalline schist one 

 may arrive at a knowledge of the composition of the original 

 rock, eruptive or sedimentary, from which that schist was derived. 

 The designation metamorphic rocks is acknowledged to be appro- 

 priate. 



The Crystalline Schists are treated under the following heads : 



(i) Gneiss, (2) Mica schist, (3) Talc schist, (4) Chlorite 

 schist, (5) Amphibole and pyroxene rocks, (6) Serpentine, 



(7) Lime series, (8) Magnesian series, (9) Iron series, (10) 

 Emery. 



Concerning these groups Rosenbusch remarks : 



In most of these large groups of the Crystalline Schists, which are held 

 together mainly through mineral composition, there are united rocksof funda- 

 mentally different genesis. Therefore they are not natural but rather artifi- 

 cial series. For the replacement of these artificial groups by natural ones 

 there is lacking, at the present time, both necessary breadth of experience 

 and maturity of judgment, from which the need for reform in various direc- 

 tions is evident. 1 



Believing that the natural classification of metamorphic rocks 

 must develop by the historical method, with the increase of 

 knowledge, Rosenbusch proposes, as a step in the desired direc- 

 tion, to apply the prefix ortho to the names of gneisses derived 



1 In den meisten dieser grossen Gruppen von Krystallinischen Schiefern, welche 

 lediglich durch gleichen oder ahnlichen Mineralbestand zusammengehalten werden, 

 sind genetisch grundverschiedene Gesteine zusammengefasst. Daher sind sie nicht 

 naturliche sondern kiinstliche Reihen. Zur Umgestaltung dieser kiinstlichen Gruppen 

 in naturliche fehlt zur Zeit noch einerseits die erforderliche Breite der Erfahrung, 

 andererseits die Reife des Urtheils und damit das Bediirfniss nach Reform in weiteren 

 Kreisen. — Elemente, p. 461. 



