244 The American Geologist. October, i9oo 
in the characters of labradorite in the two classes of rocks? 
Or is it due simply to a greater basicity in the particular frag- 
ment serving for the measure? Several other examinations of 
the labradorites of the deep-seated rocks can alone decide the 
question. 
The comparison with the theoretical angles established by 
Michel Levy for the labradorite Ab.=An4 leads to the same re- 
sults ;the angle of extinction in S//g is remarkably larger in 
the Minnesota plagioclase than the theoretical value. 
Afiglcs betivecii planes pcrpciuiicular to bisectrices and 
cleavage faces. It is only rarely possible ito determine the 
position of the optic elements by direct goniometric measure- 
ments of the angles between an artificial plane perpendicular 
to one of the bisectrices, and some of the natural planes of the 
mineral. In those cases when the natural faces are not wholly 
destroyed in the making of the artificial plane, the section is 
usually so thin that very little accuracy can be expected. The 
Minnesota feldspar has afforded only the merest approxima- 
tions. The plane S;/g, makes an angle of about 47° with 
/(ooi); whilethe surface T//p makes an angle of about 66° with 
/(ooi), and about 69° with ^'(010). No similar measurements 
have been made to the knowledge of the writer except those 
by Descloizeaux, and Fouque, which, however, are mutually 
complementary. Omitting those not accompanied by analy- 
ses, they can be presented as follows: 
Angle between Angle between 
S»g or T)*g T//), or S//|> 
and and 
Name Approximate pmY) ^(001) yMOlO) /)(001) .(/'(OlO) 
Composition DesCl Fouque Fouque Fouque Fouq\ie 
Albite AbjAno 78=-81^ 78° 89 70' 
Oligo.-Albite Abj^Ani 83° 
Oligo. -Albite AbgAn, SG-'SO' 87" 
Oligoclase Ab4Ani 78' 
Oligo. -Andesine AbgAnj 72" 74" 
Andes.-Oligo. Ab^An^ 68' 68° 
Andesine Ab^Ang 62° 
Labradorite Ab^Anj 56° 53° 31° 
Labradorite Ab^An^ 49°30' 
Anorthite Ab/Auj, 35° 50 
79-84 
74 
71 
65° 
58° 
53" 
