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



11. — On the Sand-grains in Micaceous Gneiss from the St. 

 GoTHARD Tunnel; and on some other Difficulties raised 

 BY Prof. Bonney. 



By Dr. F. M. Stapff. 



FROM the "Annals of British Geology," 1892, No. 533, p. 294, 

 I find that Prof. T. Bonney doubts the occurrence of sand- 

 grains in the Guspis micaceous gneiss,^ as stated by me in various 

 reports on the geology of the St. Gothard Tunnel, and repeated on 

 p. 17 of my "Remarks on Prof. Bonney's paper on the Crystalline 

 Schists and their relation to the Mesozoic rocks in the Lepontine 

 Alps." ^ The question being of some general interest, and not 

 only of importance for the comprehension and classification of the 

 crystalline schists in the Lepontine Alps, I think it desirable to 

 repeat here the observed facts, and to present to the notice of the 

 readers of the Geological Magazine some autotype prints of the 

 rock in question. I am the only geologist who has seen these beds 

 in situ, when they were opened by the St. Gothard Tunnel ; and 

 it would be rather difficult for anyone to make out their outcrops 

 along the line of profile through the Guspis Valley and below the 

 actual moraine of the St. Anna glacier, without taking into con- 

 sideration the faults and thrusts in this tract, which are indicated on 

 my geological map of the St. Gothard Railway and on the profile 

 of the tunnel. 



I beg leave to quote a passage from the geological records kept 

 during the construction of the St. Gothard Tunnel (Geologische 

 . Durchschnitte und Tabellen iiber den grossen Gotthard-tunnel ; 

 Spezialbeilage zu den Berichten des Schweizerischen Bundesrathes 

 ueber den Gang der Gotthardbahnunternehmung, 1874-1882 ; 

 Zurich Orell Flissli), which refers to the bed No. 130; 7262, 4 m. 

 from the northern entry (loc. cit. p. 178-9) : — 



"No. 130; 1879, Sept. 4th; ditto (fine-grained rather dense 

 gneiss) ivith sandy quartz- grains ; 7262, 4 to 7278, 3 m. ; false- 

 stratification 20 W. and 77 E. to 5 E. and 64 W., average 11 W. 

 and 82 W. Samples taken at 7262, 4. Fine-grained, rather dense, 

 micaceous gneiss, rough, almost tufaceous (but not porous), of 

 reddish-grey colour and confused parallel structure, which is 

 indicated by shreds of brown mica. In this groundmass grains of 

 quartz and felspar of the size of a peppercorn and smaller are 

 disseminated, whence the rock assumes a porphyritic structure. 

 Some crystalline outlines of dull, whitish, faintly glittering felspar 

 may plainly be recognized. The grains of glassy and milky 

 quartz show an iridescent, glossy, conchoidal fracture ; they are 

 rounded, with some crystal facets here and there ; enveloped by 

 thin mica-films (macroscopically visible), which can readily be 

 loosened and picked out fi-om the groundmass. The presence of 

 calcspar is indicated by effervescence, especially around the quartz- 

 grains ; other accessory minerals are : pyrites and rarely minute 

 fragments of tourmaline. 



1 See Correspondence, Geol. Mag. 1892, p. 90. 



^ Geological Magazine, January, 1892, Dec. III. Vol. IX. pp. 6-21. 



