Br. F. M. 8tapjf — Sand-grains in Micaceous Gneiss. 153 



Microscopic observations. — The contours of the larger quartz- 

 grains (as far as they have not been broken out by the grinding 

 operations) are rough but not splintery ; the grains are partially 

 "wrapp)ed in some longer pellicles or shorter split scales of brown 

 mica. Other grains are surrounded by a garland of fine mica films, 

 which occasionally seem to extend into the quartz-grains. Calcspar, 

 together with occasional quartz splinters, is deposited in these mica 

 envelopes, and is attached to the sandy quartz-grains without visible 

 traces of abrasion. Each sandy quartz-grain forms a single in- 

 dividual, which is almost always fractured, with or without slight 

 dislocation and torsion of the fragments. The cracks are often 

 covered by a brownish-green substance. In the interior of the 

 quartz-grains may be observed some microscopic inclusions of opaque 

 dust, specks of pyrites, some of mica, a few small lacunce 

 (pores), and some short thin colourless needles. Here and there 

 a fern-like figure is dimly seen, perhaps indicating the face of a 

 microscopic crack, rather than any microlithic material. Most of 

 the plagioclastic felspar is twinned, and the fine twin-striations are 

 visible (in polarized light) though the felspar is in a state of advanced 

 decomposition and intermixed with clusters of mica scales, which 

 veil the striations. Though the outlines of the felspars are not so 

 distinct as those of the quartz-grains, they are yet better defined 

 and more clearly individualized than those of the ordinary crushed 

 felspars in the micaceous gneiss ; and thus they convey the im- 

 pression of being porphyritic inclusions rather than indigenous 

 constituents of the micaceous gneiss. The groundmass surrounding 

 the grains of quartz and plagioclase is a compressed mixture of 

 quartz, felspar and two hinds of mica, the films of which induce an 

 indistinct parallel structure. A little calcspar also makes its 

 appearance, partly in isolated granules, partly in larger groups of 

 polysynthetic twins, which are cross-barred with the other minerals, 

 so that it sometimes appears to be a constituent mineral, and some- 

 times a later-formed accessory cement. Microscopic accessories 

 are pyrites (also pyrrhotine) in irregular masses, grains, and small 

 crystal groups ; and minute, but easily recognized, fragments of 

 tourmaline. (So far microscopy.) 



This rock appears to be one of the most curious from the in- 

 terior of the St. Gothard massif. If it really be psammitic, as one 

 may suppose from the foregoing, then all the stratified beds bored 

 through in this massif must also be of sedimentary , origin. It 

 recalls to mind the sericite slates of the Ursern Valley, with their 

 glossy sand-grains ; a similar, though less characteristic, rock 

 occurs at 5560-5600 m. from the southern entry. On newly- 

 opened wall-surfaces in the gallery, the quartz-grains appeared to 

 be arranged like pearls in strings (scarcely visible in hand-specimens); 

 the strings did not agree with the parallel structure of the rock, but 

 rather with its false-bedding (or secondary schistosity) ; and hence 

 the interpretation of this feature, and of the rock itself, becomes still 

 more complicated. At its margins the bed No. 130 is penetrated by 

 compact veins of quartz and felspar, and further on it passes over 

 into the ordinary micaceous gneiss " 



