Gabbroid Rocks of Minnesota. — Winchell. 227 
same cleavage on the labradorite from the other locahties from 
which it has been possible to examine material. Over one 
hundred cleavage crystals were carefully examined on the 
goniometer and all those showing good reflections were 
measured. Of the results obtained a few were discarded as 
certainly erroneous since they showed a wide variation from 
any published value. The others are as follows, including a 
few other angles which were measured merely incidentally as 
they presented themselves: (see p. 228.) 
What is the cause of the variations, which are nearly as 
wide in the single series of measures, not only from the same 
locality, but from the same hand specimen, as those shown by 
the table of measures by various authors from widely separ- 
ated localities? They are certainly not due to the apparatus 
used, as it has been repeatedly tested with other minerals, and 
found very satisfactory; neither are they entirely due to the 
inexperience of the operator, since one of the most exceptional 
*Rose: Gilbert Ann. LXXIII. 1823, p. 194. 
fHessel: Kastner's Archiv. X, 1827, p. 274. Ref. Schrauf: Sitzb. 
Akad. Wissensch. Wien. LX. 1869. 
JNordenskjold: Pogg. Ann. XIX, 1830, p. 181. Measurements 
made with Wollaston goniometer. 
§Reusch: Pogg. Ann. CXX, 1857, p. 96. 
llMarignac in Des Cloizeaux. Min. 1862, p. 303. 
^Vogelsang: Archives Neerlandaises III, 1868. 
**Schrauf: Sitzb. Akad. Wissensch. Wien. LX. 1869, p. 996. 
tfvom Rath: Pogg. Ann. CXLIV. 1871, p. 219. 
Jivom Rath: Ber. Akad. Berlin. XLI, 1876, p. 147. 
§§Tschermak: See Dana Min. 1892, p. 326. 
llliObermayer Zeits. Kr. Ill, 1893, p. 66. 
til Holland: Min. Magazine. London. 1889. VIII. p. 154. 
***F. Fouque: Bull. Soc. Fr. Min. XVII. 1894, p. 343. 
tttNaumann and Zirkel: Elem. Min. 1898, p. 730. 
JJJNaumarn and Zirkel: Elem. Minn. 1898, p. 721. 
§§§Several analyses from this locality show wide variations, so that 
the composition of the material measured is uncertain; I have been 
imable to find any evidence thatjhe material measured was analyzed. 
However, the measure of w//(i 10) A (i 10) = 121° 2,"]' is said to have 
been on feldspar of the composition AbsAui. 
jlllllThe value is from the measurement pp'(ooi) A (001) = 172°. 
^TITl These values are also from measurements of twinning faces. 
****Material from this locality gave Segeth the composition Ab? An, 
nearly. It is remarkable that Obermayer gives only four measures: 93" 
4.3'. 93° 44'. 93° 48' and 93° 50', and then concludes that the true value 
IS 93' 52'! And this value, which was not once actually observed by 
Obermayer, is the basis of the axial elements as given by nearly all 
the best mineralogies of the present time. 
