180 BULLETIN OF THE 
the minerals outside. The centre, however, is cloudy, owing to fluid 
inclusions and particles of limonitic material. Compared with the cases 
previously described, the cloudy area is of less importance ; it not only 
occupies a smaller portion of the whole grain, but the inclusions are not 
in such close aggregates ; there is also no difference in polarizing tint 
between the clear and cloudy portions. : 
In Figure 8, already referred to, we have the type of the pure 
“albite” variety of feldspar; namely, a homogeneous area of feldspar 
without a linear boundary, having a somewhat longer dimension parallel 
to the schistosity of the rock, glassy clear, and polarizing with a low 
tint. In this case the black inclusions are magnetite, the others flakes 
of muscovite. Muscovite bounds the grain on either side. 
In order to determine more accurately the nature of these feldspars, 
a portion of the rock was powdered and a separation of the constituents 
made by the Thoulet solution. From the powder thus obtained slides 
were prepared by scattering a little of each powder in balsam on a glass 
slide and cautiously grinding down to the required thinness; the thin 
sections thus obtained could be examined microscopically nearly as well as 
in ordinary slides and with a predominance of cleavage sections. With 
the first falling, the muscovite, magnetite, and tourmaline came down. 
Between the specific gravity of anorthite, 2.76, and that of quartz, 
2.65, a little material was obtained, which was found to be feldspar and 
quartz weighted by mica or magnetite as impurities. At 2.65, the bulk 
of the powder came down, which was found to be quartz; between this 
and 2.60, the lower limit of plagioclase, a considerable quantity of pure 
feldspar came down, which in the slides exhibited the properties of what 
has been described above as the albitic feldspar; that is, the grains 
are untwinned or simply twinned, clear and glassy, with occasional 
muscovite or quartz inclusions. Sections could be found cut parallel to 
the basal cleavage, twinned in two single halves and giving an extinc- 
tion 4° oblique to the second cleavage, while other sections cut appar- 
ently parallel to the second cleavage have an extinction 17° oblique to 
the first cleavage, and show a bisectrix slightly oblique. Without 
chemical analysis this is as complete a determination as was possible, 
and by Sp. Gr. and optical properties indicates albete. With this albite 
there occur some grains of a multiple twinned plagioclase, and some 
microcline apparently weighted by inclusions. 
Another feldspathic portion of the powder was obtained between 2.60 
and 2.56, and the larger portion of this was microcline. A part was of 
the fresh glassy variety, free from inclusions, with distinct double twin- 
