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 

 pi'eviously described, the cloudy area is of less importance ; it not only 

 occupies a smaller portion of the whole grain, hut 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 ; nameh', a homogeneous area of feldspar 

 without a linear boundary, having a somewhat longer dimension pai-allel 

 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 cither 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 dow^n, 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° obhque 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 alhite. 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- 



