146 



METAMOBPHIC EOCKS 



small grains and show under the microscope a mosaic structure. 

 The Saxon granulites are regarded by Lehman as eruptive 

 rocks altered by pressure. Ealleflinta is a Swedish name for 

 a rock resembling in most respects the eruptive f elsites or quartz 

 porphyries already described. Porphyroid is also a felsitic rock 

 with a more or less schistose structure, and with porphyritic 

 feldspar or quartzes. 



Inasmuch as the structure characteristic of gneisses is found 

 developed in rocks of diverse types, many petrologists now use 

 the term in an almost wholly structural sense, as in itself non- 

 committal as to composition or origin, but merely designating a 

 rock of foliated or schistose structure. C. H. Gordon has pro- 

 posed^ a scheme of classification of gneissoid rocks as below 

 which has much in its favor. 



Classification of Gneiss 



Asjlzoqovb Massivb Typb 



Of Ignkous Orighn 



OEiaiN Unknown 



Granite : 



Granite gneiss : 





Granitic gneiss : 



Biotite granite . . . 



Biotite granite gneiss 



( . 



Biotite granitic gneiss. 



Hornblende granite . . 



Hornblende granite 



} 



Hornblende granitic 





gneiss .... 



gneiss. 



Syenite : 



Syenite gneiss : 





Syenitic gneiss : 



Hornblende syenite . . 



Hornblende syenite 



} 



Hornblende syenitic 





gneiss .... 



gneiss. 



Mica syenite . • , . 



Mica syenite gneiss. 



* 



Mica syenitic gneiss. 



Pyroxene syenite . , 



Pyroxene syenite gneiss 



Augite syenitic gneiss. 



Diorite : 



Diorite gneiss : 





Dioritic gneiss : 



Micadiorite .... 



Mica diorite gneiss . 



* 



Mica dioritic gneiss. 



Gabbro ....... 



Gabbro gneiss . . . 



• 



Gabbroic gneiss, or gab- 

 bric gneiss. 



Pyroxenite 



Pyroxenite gneiss . . 



• 



Pyroxenitic gneiss. 



(2) THE CEYSTALLINB SCHISTS 



Under this head are grouped a large and extremely variable 

 series of rocks, differing from the gneisses mainly in the lack of 

 feldspar as an essential constituent. They consist, therefore, 

 essentially of granular quartz, with one or more minerals of the 

 mica, chlorite, tale, amphibole, or pyroxene groiip. In acces- 



^Bull. Geol. Soc. of America, YoL Yll, p. 122. 



