Eggleston — Eruptive Rocks at Cutting 'sville, Vt. 399 



sumably due to the oxidization of iron sulphide or some 

 other substance disseminated through the feldspar. The 

 rock yields a strong sulphurous odor while being broken. 

 Out-door weathering rusts the rock surface, finally 

 changing the color to pale buff or very light brown. 



In thin section the texture is holocrystalline, hypidio- 

 morphic granular. Seventy-five per cent or more is 

 feldspar, of which microperthite is most notable and is 

 accompanied by microcline, probably anorthoclase, ortho- 

 clase, and a little albite. Green augite, locally rimmed 

 with green hornblende, is chief of the remaining min- 

 erals. With it there is some aegirite-augite whose 

 extinction angle approaches that of segirite. An occa- 

 sional large grain of olivine (fayalite?), more or less 

 serpentinized, and a little primary quartz, in distinct 

 grains, are usually present. Magnetite, possibly some 

 ilmenite, zircon, and apatite are accessory. A single, 

 large crystal, possibly allanite, was noted in one section. 



The mineralogical composition indicates that the rock 

 is an augite syenite of the nordmarkite group, a conclu- 

 sion borne out by a comparison of its chemical analysis 

 (column I of the following table) with other nord- 

 markitic types and with average nordmarkite (col- 

 umn VII). 



ii 



in 



IV 



V 



VI 



VII 



SiO a 



67-30 



65-43 



65-43 



60-45 



6613 



67-99 



64-36 



TiO, 



0-17 



016 



0-50 





0-74 





0-45 



A1A 



17-44 



16-96 



1611 



20-14 



17-40 



17-54 



16-81 



Fe,0 3 



1-56 



1-55 



115 



3-80 



219 



1-17 



1-08 



Feb 



1-76 



1-53 



2-85 



not det. 



not det. 



0-82 



2-71 



MnO 



016 



0-40 



0-23 





013 





015 



MgO 



0-14 



0-22 



0-40 



1-27 



004 



013 



0-72 



CaO 



100 



1-36 



1-49 



1-68 



0-81 



1-44 



1-55 



Na,0 



7-08 



5-95 



5-00 



7-23 



5-28 



4-92 



5-76 



K,6 



4-78 



5-36 



5-97 



512 



5-60 



5-78 



5-62 



H,0 + 



005 



0-82 



0-39 



0-71 



1-22 



005 



0-70 



HoO — 



005 





019 











PA 



004 



002 



013 









009 



CO. 





None 



Trace? 











Zr6. 



008 





011 











so 3 



None 



006 



None 











Cl 





004 



005 











F 







008 











BaO 





None 



003 











SrO 







Trace 











FeS. 







007 











LiO 







Strong trace 









Sum 



101-61 



99-86 



100-18 



100-40 



99-54 



99-84 



10000 



Specific 



gravity 





2-659 











