GEOLOGY OF MOUNT MARCY 3I 



Mol. ratio 



Si02 SO. 33 .838 ) o_oo 



TiO. .07 .0008 f -^^ 



AUOs 3.36 Q,^ ^ 



Fe.03 1.03 .oS [ -039 



FeO 19.40 ^ 



MnO .71 -^'^ I Q „ 



M-0 21.40 -^^^ f -8^7 



CaO 2.77 -535 J 



H.O i.il -O^Q 



Z .063 



100.21 



Obviously the mineral contained chiefly MgO.SiO^, FeOjSiOg, 

 with a little MnOjSiO,. Presumably the CaO was present in some 

 monoclinic pyroxene or in some admixed labradorite, combined with 

 the alumina and silica. 



The next dark silicate in relative importance, if indeed it does not 

 exceed the hypersthene in actual frequency, is augite, which is 

 emerald green in thin section. It is illustrated in plate 15B. Pre- 

 sumably the same mineral is described by Professor Leeds under 

 the name diallage, a name now much less used than formerly. His 

 analysis follows. 



Mol. ratio 



} 778 



682 

 596 



100.065 



It is evident from the molecular ratios that we have an excess of 

 silica no matter what well-known pyroxenic molecules we assume. 

 If we recast by assigning all the alumina to the anorthite molecule so 

 as to use twice as many silicas, we are still confronted with an excess. 

 There must have been some free silica in the sample in order to con- 

 form with the percentages, or else some included feldspar with the 

 albite molecule, whose soda was not determined. 



Hornblende has been occasionally noted in the anorthosites and 

 biotite also. Biotite is not often seen in the Mount Marcy quad- 

 rangle. It is far more abundant in the anorthosites of the south- 

 eastern portion of the Ausable quadrangle. Titaniferous magnetite, 

 the mechanical intergrowth of magnetite and ilmenite, is scattered at 

 times through the anorthosites in association with the bunches of 

 dark silicates. No great masses of it have been discovered in the 



Si02 



46.28 



.771 



TiO. 



• 59 



.007 



AUO3 



7.38 



.072 



Fe.iDa 



2.21 



.014 



FeO 



14.80 



.206 



MgO 



8.91 



.222 



CaO 



18.78 



.168 



H.O 



1. 115 





