510 H. IV. TURNER 



6. Websterite. — Oakwood, Cecil county, Md. Composed of 

 hypersthene and diallage. W. F. Hillebrand, analyst. Bull. U. 

 S. Geol. Survey, No. i68, p. 43. 



7. Composite pyroxenite . — Hobbs, Jour. Geol., Vol. VIII, p. 30. 

 This analysis is a composite from three analyses of pyroxenites, 

 and one analysis of a hornblende-hypersthene rock from Gallatin 

 county, Mont. 



Professor J. S. Diller, in his bulletin on "The Educational 

 Series of Rock Specimens,"' introduces three specimens which 

 would fall into the perknite group. 



No. 1 10, a pyroxenite, is described by Professor George H. 

 Williams. 



No. Ill, feldspathic peridotite, is described by Professor 

 George H. Williams. 



No. 113, Cortlandite (hornblende-peridotite), is described 

 by Williams and Iddings. The rock is composed of brown 

 hornblende, olivine, pyroxene, biotite, feldspar, and magne- 

 tite. 



Some rocks that have been termed wehrlite will fall also into 

 this group. 



EFFUSIVE AND DIKE ROCKS 



Corresponding to the plutonic group of perknite there 

 undoubtedly occur effusive and dike rocks. Professor Rosen- 

 busch regards hornblende-picrites as effusive, and some of these 

 have the chemical and mineral composition of perknite. This 

 will also be true of augitites, since in these augite or monoclinic 

 pyroxene is the chief constituent. 



Professor J. P, Iddings has been kind enough to criticise the 

 above paper and calls my attention to the fact that my definition 

 of perknite would bring into the group the kimberlite of Ken- 

 tucky with 9.46 per cent, of lime, and the kimberlite of South 

 Africa with 9.60 per cent, of lime, as wellas the amphibole-peri- 

 dotite of Schriesheim with 7.22 per cent, of lime. He also calls 

 attention to the fact that with hypersthene-enstatite rocks there 



■Bull. U. S. Geol. Surv., No. i68. 



