Dr. F. M. 8tapf — Sand-grains in Micaceous Gneiss. 159 



crystals, but sometimes they are rounded ; or they are split, broken, 

 and torn, as felspars in the squeezed St. Gothard gneisses so often 

 are ; and hereby we learn that the same felspar simultaneously 

 makes its appearance as an ordinary constituent of the micaceous 

 gneiss and as a porphyritic inclusion in the same. 



The figured grain is porphyritic though rounded. It shows no 

 twin stripes in polarized light, and its mass is almost replaced by 

 clusters of pellicles of potash mica and Icaolin. Small quartz- 

 grains and other microliths accompany the mica in the decomposed 

 felspar, which I need not further mention. 



The outline of this grain of felspar — and of others similar — is not 

 so well defined as that of the quartz-grains (Fig. 3), and to all 

 appearance it has coalesced with the surrounding materials ; a white 

 margin indicating in some cases the zone of intermixture. 



In spite of the roundish contours of many of the felspar-grains, 

 I am not inclined to consider them to be grains of felspar sand. 

 The occurrence of porphyritic segregated felspar in gneissic rocks 

 is by no means unusual ; and this may be the case in the quartzitic 

 micaceous gneiss No. 130, where the lesser part of the felspar is 

 intermixed with the other constituent minerals, and the greater is 

 segregated. 



The preceding observations put beyond every doubt that the 

 quartz-grains are foreign bodies : their material is not identical with 

 that of the constituent quartz of the micaceous gneiss, nor are there 

 any transitions between the two kinds ; each quartz-grain is an 

 individual, loosely imbedded in the groundmass and easily picked 

 out of it. Some of the apparent inclusions in the quartz-grains 

 are later depositions on cracks, others are squeezed in Jrom the out- 

 side, and with regard to the nature of others some doubt may be 

 reasonably entertained. The question now is : have we to deal 

 with sand-grains imbedded in an (originally) sedimentary psammitic 

 rock; or with a kind of quartz-porphyry, the groundmass of which 

 has been decomposed in such a way that it is now a micaceous 

 gneiss ? This second view is supported by the nature of the quartz- 

 grains, and their association with porphyritic felspar-grains ; whilst 

 it is contradicted by the actual nature of the enveloping magma, by 

 the lack of fluidal structure, by the arrangement of the grains in 

 accordance with the bedding of the rock (Fig. 1) — facts which 

 corroborate the belief in the sedimentary origin of the bed. 



Prof. Bonney doubts also " the occurrence of organisms in the 

 Althirche marble " ; but I question whether many share his opinion 

 after seeing the autotype figures of the bodies in question, which are 

 published in the Geological Magazine (1892, Dec. 111. Vol. IX. 

 p. 16),' with a footnote by the Editor (p. 17) : " hand -specimens 

 of these St. Gothard rocks are preserved in the Mineralogical Col- 

 lection of the British Museum, and sections taken from the one 

 marked No. 43 show precisely similar structures to those in the 

 accompanying figure. — Edit. Geol. Mag." 



^ Reproduced by Professor J. F. Blake, in his Annals of Geoloffv for 1892, 

 p. 293. 



