THE 
AMERICAN GEOLOGIST, 
Vol. XXVI. NOVEMBER, 1900. No. 5 
MINERALOGICAL AND PETROGRAPHIC STUDY 
OF THE GABBROID ROCKS OF MINNESOTA, 
AND IVIORE PARTICULARLY, OF 
THE PLAGIOCLASYTES. 
[Continued. See plate.s ia the September and October XuiiibiTs. | 
By ALElt.-VNDER N. WiNCHELL, Butte. Mont. 
Chapter IV (Continued). Pla,<;ioclas\-tc'. 
ChcDiical Conipositon of Labnidoritc. The analysis was 
made on pieces carefully selected, several having served for 
goniometric measures. All the fragments floated in a liquid 
of the density 2.709 and sank in one of the density 2.696; the 
average density of glass clear grains was 2.701; the whole 
was then carefully examined with the microscope and all im- 
pure grains mechanically removed. But in the final powder 
used some traces of hematite still remained. The main analy- 
sis was executed by the Deville method; the alkalies were de- 
terniined by the Lawrence Smith method. 
The composition of the lal)radorite studied is therefore very 
nearly AbsAm; the analysis indicates that it is a trifle more 
basic than this, but the differences arc wholly negligible. In 
specific gravity the mineral is a shade lighter than the theoret- 
ical compound. The iron oxide is present in the form of hem- 
atite very finely divided, so that the iron is in the form of the 
sesquioxide; only a trace at most of ferrous iron is present. 
The composition of the mineral is nearly the same as that of 
the rock. (See plate XIII, figures 7 and 11). 
