Dr. F. M. Stapff — Sand-grains in Micaceous Gneiss. 157 



matic quartz-splinters. The tbin, colourless, brilliant prismatic 

 needles — not visible in figure — and the rhombic sections/ seemed 

 to offer a special character since they were missing in the mag- 

 matic quartz-splinters ; but later I found them also in the holes 

 from which the quartz-grains had been broken out (abundantly), 

 and finally outside the edge of the thin slice (rarely) ; whilst they 

 were wanting in another freshly-made microscopic section. They 

 seem not to he connected loith the material of the roch. Some few 

 microscopic scales of brown mica are deposited on the crachs in 

 the quartz-grains ; they appear to be later deposits and not 

 inclusions in the crystallizing quartz. But some doubt may 

 arise with regard to rare microscopic patches of brown mica and 

 quartz, i.e. of the enveloping material, which appear to be inclusions, 

 though they may quite as well be deposited above or below the 

 grains, or they may be terminal points of lateral inclusions in bays 

 of the quartz-grains (see figure). Small (microscopic) granules of 

 calcspar likewise occur on cracks traversing the quartz-grains ; and 

 without doubt they have been deposited there a posteriori, just 

 as well as the calcspar on the margin of the grains. The same 

 cannot so decidedly be said about the granules of calcspar, which 

 are interwoven in the texture of the micaceous gneiss ; the less 

 so, as they proved to be dolomitic. 



The shape of the quartz-grains is always roundish polygonal; 

 short, straight fragmentary edges of a crystal are interrupted or 

 connected by curves and small fractui'e-lines ; but the outline is by 

 no means fringed by filaments or threads entering and losing them- 

 selves in the enveloping material. No regularity whatever can be 

 recognized in the crystallographic orientation of the grains, either 

 with respect to each other or towards the surrounding minerals ; 

 the different quartz-grains exhibit very different orientations and 

 individual colours when the slice is moved between unaltered 

 Nicols. And just as little can any fiuidal structure be noticed, 

 around the grains. Short segments are wrapped up in extended 

 pellicles of mica, which stick close to the outline of the grain and 

 leave but little room for the deposition of other mineral particles ; 

 for the rest, small brown mica scales intermixed, to form a minute 

 mosaic, with broken quartz (and some grains of felspar, calcspar, 

 etc.) environ the foreign quartz-grains; and it is seen that the mica 

 pellicles are twisted around the quartz-grains in such a way that 

 longitudinal and cross sections alternate in one and the same profile. 

 Hence the cavity from which a quartz-grain has broken out seems 

 to be lined by a cuticle of brown mica ; but, if that same cuticle 

 is cut through, the sections vary very considerably at different 

 places. 



The demarcation line between a quartz-grain and its envelope is 

 seldom quite clean and smooth. Microscopically seen it is, for the 

 most part, rough, and small pellicles of mica and granules of quartz 

 are attached to it (squeezed in, to all appearance) ; just as we see 



1 Mentioned in the translation from the Geol. Durchschnitte und Tabellen. 



