PVROXENITES OF THE G RENVILLE SERIES 325 



"pyroxene-gneiss" for the French equivalents of the pyroxenic 

 rocks occurring in beds or nests in the Grenville series, and con- 

 tends that the Canadian use of the term "pyroxenite" for these 

 rocks should be abandoned and the name restricted to non-feldspathic 

 plutonic rocks free from olivine. The term is thus made a class 

 designation co-ordinate with "peridotite," given by Rosenbusch to 

 the corresponding olivine-bearing series, and the author says that 

 its use as a designation for any rocks except those of igneous origin 

 should be abandoned. As nearly all the Canadian pyroxenites were 

 regarded by Williams, evidently on the authority of Hunt and others, 

 as metamorphosed sedimentaries, they were excluded from the list. 

 Zirkel^ adopts Williams' classification, and includes these rocks with 

 Lacroix's wernerite rocks under the pyroxene gneisses of the crys- 

 talline schists, apparently, though without distinct reference. 



Pyroxenic rocks with augite as the chief basic mineral are not 

 of common occurrence. Clements has described one from Alabama^ 

 which seems to be closely allied to these Canadian pyroxenites. In 

 a comparison of the Ottawa pyroxenites with younger rocks, mani- 

 festly consideration must be given to the long period of time during 

 which the former have been subjected to mountain-making forces. 



As the result of this study we conclude: 



1. That these pyroxenic rocks of Ottawa county were intruded 

 into the overlying sedimentary beds at considerable depths, and 

 solidified originally as a coarse-grained pyroxenite consisting chiefly 

 of augite, but approaching the gabbro end of the series. 



2. That, through the changes effected by pressure and metaso- 

 matic processes, w^hereby original structures have in large part dis- 

 appeared, the rock should now be classed with the pyroxenite- 

 gneisses. 



3. That the apatite deposits associated with these rocks were 

 due to fumarolic action at the time of the intrusion of the pyroxenites, 

 and also, to a greater extent possibly, to that attending the later intru- 

 sions of syenite. 



C. H. Gordon. 



University of Washington. 



' Lehrhuch der Petrographie, Vol. Ill, p. 149. 



2 Bulletin 5, Geological Survey of Alabama, p. 163. 



