18 Br. F. M. Stapff— Crystalline Schists 



as Piottino ' and Tessiner gneiss ; and also equivalent to the gneiss in 

 the neighbourhood of Erstfeld, north of St. Gothard (pi. i. of map). 

 In the tunnel this gneiss with its varieties and occasional intercala- 

 tions of micaceous and amphibolic rocks, occupy about 1000 metres 

 (4000—5000) on the south side, and MOO m. (6000—7400) on the 

 north side, but it should be observed that frequent repetitions and 

 transitions of the terminal beds tend to make these limits rather 

 elastic. The Sella gneiss II. is a highly crystalline felspathic rock, 

 containing both potash- and magnesian-mica, and it has a veiny, 

 uneven or lamellar, glandular structure. 



3. The Sella gneiss is succeeded by the micaceous gneiss III., 

 named after the Alpe Sorescia on the south side, and after the 

 Gnrschen Alp on the northern side. It occupies in the tunnel 

 800 m. (3200-4000) on the south, and 1700 m. (4300-6000) on the 

 north side. The predominant mica is brown or grey magnesian ; 

 that of the north side often assumes a green colour, and from this 

 and from the occurrence of sericite it passes over into Ursern gneiss, 

 whilst accessory garnets and hornblende in its boundary beds on the 

 south side indicate a relation to the felspathic mica schist of the 

 Scipsius. Moreover the rocks of this series have more frequently 

 the character of mica schist than of gneiss. 



The gneissic series III. is better characterized from a geological 

 than from a petrographic standpoint. The peridotic and pyroxenic 

 serpentines of the St. Gothard make their appearance in it, as also 

 certain of the hlaclc garnet schists and some insignificant ore 

 deposits. The serpentines which crop out along the Ursern valley, 

 between the Unteralpthal, Gige, Hospenthal, and Zumdorf (pis. iii. 

 and iv. of geol. map) were passed in the tunnel between 4870 and 

 5310 m. N. ; and an analogous series of lenses of serpentine has 

 been followed on the south side, in the same micaceous gneiss, on 

 both slopes of the Val Tremola, near Seara Orell and Fieudo (pi. v. 

 of geol. map), though not met with in the tunnel. Beyond the 

 northern slope of the St. Gothard on the other side of the granitic 

 Finsteraarhorn gneiss, diallagic (?) rock appears near Gurtnellen and 

 down on the railway line near Meitschlingen in the same series III. 

 (or IV.?). Finally, we find serpentine south of the Gothard in the 

 neighbourhood of Bellinzona, at Castanetta, E. of the railway, and 

 at Seraentina W. of it — in both cases in micaceous gneiss belonging 

 to III., which also is the prevailing rock of Mount Ceneri and in the 

 Agno valle}'^ downwards to Gravesano and Manno, where it meets 

 the Palaeozoic rocks. (Profile on title-sheet of map.) The black 

 garnet schists of the Unteralpthal and Val Cadlimo (Lagoscuro), which 

 are imbedded in the micaceous gneiss III. have been mentioned 



1 " Piottino gneiss " means the uppermost beds -which on Mount Piottino (Dazio- 

 grande) dip under the mica schist formation. Tessiner g-neiss is an old term of 

 Studer's, which refers to the nearly horizontal strata of gneiss along the Ticino, 

 Near the railway station Claro they abruptly assume a sharp southward dip, and 

 are then overlaid by a newer gneiss formation (See pis. ix. and x. of the miip and 

 Explanation of the same in Zeits. d. deutsch. geol. Gesells. 1884, vol. xxxvi., also 

 Neues Jahrb. f. Mineral., 1882, vol. i. p. 72, where also the relation between the 

 parallel structure and the stratification of the Piottino gneiss is treated of. 



