Gabbroid Rocks of Minnesota. — \ Vinchell. 237 
7. Labradorite, pg^ (001) i\ ioio) = 94° 16', as determined by the 
writer on feldspar from Carlton peak, Minnesota, of the composition 
Ab3An4, nearly. (See above.) 
8. Labradorite,. /^Hooi)A (010) = 94° 53', average of the three 
determinations which have been made hitherto on material of the 
composition Ana Ans, nearly. (See above.) 
9. Labradorite, /^^^ (001) A (010) ^ 93° 38', determined by Hol- 
land on m^aterial from Hull, one of the Hebrides islands, of the com- 
position AbjAn^. nearly. (See above.) 
10. Anorthite /^^ (001) A (010) = 94° 10', determined by Marig- 
nac and Des Cloizeaux on crystals from Vesuvius; chemical composi- 
tion not given; result confirmed by Koksharow and universally ac- 
cepted. 
Since no measures have been made hitherto (so far as 
known to the writer) on the feldspar Ab3An4, it may be of in- 
terest to present the result of various angles and axial elements 
calculated on the basis: /^'(ooi) A(oio)=94° 16', //(no) A 
(oio)=i20° 48'; ;/2/^^iio) A(iio)=i2i° 15'; and// (ooi)A 
(iio)=:^ii4° 16': 
a : b : c = .63314 : 1. : (..5+) 
a =yz = 93°20i^ ' A = X = 9*«16' 
yg ='zx = 11.5°48 ' B = Y = IIS'^SS ' 
^ = xy = 90"30 ' C =- Z.= 91°51 ' 
<-i//4010)A(liO) = 117«59' ;///zi(liO)A(100) = 153^52' 
^irV2i(010)A(100) = 9P51' ///MllO)A(lOO) = 151°3' 
^n^KOlO) A (150) = 160''41 ' p/i ' (001) A (100) = 1 15°52 ' 
;/^/(flO) A(llO) = 12P13 ' /;/^(001) A(llO) = lir31 ' 
/^|(001)A(150) = 94012' 
/icr-KOOl) A(150)^ 102^19 ' (measured = 102«6 ') 
// ' (001) A (001) =- 171«28 ' (measured = 171°25 ') 
Optic properties of labradorite. The refractii'c indices 
were determined by the method of total reflection with a Ber- 
trand apparatus. Much to the regret of the writer, it must be 
admitted that the limit of error is as high as .002, on account 
of defective apparatus. The results follow: 
"s = 1-5705 
//,„ = 1.5660 
N = 
= 
1.5626 
.0045 
.0034 
direct 
measure = 
.0046 
.0036 
+ 
.0079 
Up 
.566 
