Eggleston — Eruptive Rocks at Cutting 'sville, Vt. 397 



nephelite. Minute grains and small masses of pyrite are 

 disseminated through the rock. Blue sodalite is rarely 

 discernible in the hand specimen. 



Thin sections show about 90 per cent feldspar, of which 

 upwards of one-half is microperthite, orthoclase, and 

 microcline. Both the orthoclase and microcline are some- 

 what microperthitic. Some form of microperthite is thus 

 the chief feldspar. A part is possibly anorthoclase. 

 The plagioclase is near albite in composition. Colorless 

 sodalite forms angular and veinlike areas between and 

 penetrating the feldspars. The chemical analysis indi- 

 cates its amount as about 3 per cent. The nephelite, with 

 nearly equal abundance, is in various stages of alteration, 

 to cancrinite, zeolites, and muscovite. Pyroxene is next 

 to the feldspars in abundance. It is a pleochroic variety, 

 the color ranging from pale yellowish green through 

 olive-green to green. Extinction angles indicate an 

 Eegirite-augite somewhat nearer segirite than augite. 

 There is some biotite bordering, penetrating, and 

 included in the segirite-augite. 



The order of crystallization is given in the following 

 list, which shows also the nature of the acessories: (1) 

 apatite; (2) magnetite, titanite, and possibly pyrite; (3) 

 biotite; (4) segirite-augite; (5) plagioclase; (6) soda- 

 lite, nephelite, and most of the feldspars. 



The chemical analysis (column I of the following table) 

 agrees with the mineralogical composition in referring 

 the rock to the sodalite-nephelite syenites. For con- 

 venience the analyses of other types are also entered: 



ii 



in 



TV 



VI 



SiO, 



60-88 



56-45 



60-20 



6013 



58-61 



61-86 



TiO.. 



0-35 



0-29 



014 



115 



110 



015 



ALA- 



20-75 



20-08 



20-40 



20-03 



21-12 



1907 



Pe 2 3 



0-63 



1-31 



1-74 



2-36 



2-62 



2-65 



FeO 



113 



4-39 



1-88 



1-33 



1-14 



1-49 



MnO 



0-22 



009 



Trace 



Trace 



Trace 



001 



MgO 



0-17 



0-63 



104 



0-76 



0-79 



0-55 



CaO 



108 



2-14 



2-00 



0-87 



0-62 



1-47 



Na„0 



6-35 



5-61 



6-30 



6-30 



7-85 



6-45 



K,6 



5-57 



7-13 



607 



5-97 



5-93 



5-75 



HoO + 



0-88 



1-51 



0-23 



1-41 



101 



0-47 



HX> — 



003 



0-26 



0-10 



016 







P,0 5 



Trace 



013 



0-15 



006 



Trace 



008 



CO, 



0-61 





None 



None 







ZrO, 



Trace 





Trace 



005 







so 3 



None 





013 



014 







CI 



018 



0-43 



009 









