of the Lepontine Alps. 9 



presumptively belong to the same geological age, and at present their 

 presumed horizons are rather guessed at than accurately determined. 

 But the hlach schists which are easily recognized, and in which 

 fossils may be expected to occur, will one day prove valuable criteria 

 for geological classification, and I have therefore indicated them, 

 whei'ever they occur, not only in the profile of the Gothard tunnel, 

 but also in the " Geologische Uebersichtskarte der Gotthardbahn- 

 strecke Kil. 38-149 (Erstfeld-Castione) ; 10 Blatter im Maasstab 

 1:25000; im Auftrag der Direction der Gotthardbahn, 1885." In 

 the title and index-sheet of this map, three distinct designations are 

 given for at least four different sorts of black schists ; the colours 

 referring to similar petrographic characters, the letters and numbers 

 to geological position. In this way (which, however, cannot be 

 fully explained without the help of a descriptive text to the map) 

 I have endeavoured to bring the beds together in the order in which 

 they might be expected to succeed each other, without prejudicing 

 the final geological grouping, or making the map useless in case 

 some of my views respecting the geological position of some of the 

 beds should subsequently prove erroneous. 



II. The gray mica schists, cah-mica schists, disthene schists. — I have 

 divided the rocks of the TiLino valley from the mouth of the tunnel 

 inwards to the micaceous gneiss of the Gothard, into the following 

 four groups : 



37 — 90 m. ; characteristic rocks : dolomites. 



90 — 1142 ra.; ,, ,, : gray garnet-mica schists. 



1142 — 1833 tn. ; ,, ,, : green and black garnet-mica schists. 



1833 — 3178 m.; ,, ,, -. felspathie-mica schists and amphibolic rocks. 



Prof. Bonney's " Val Piora schists " belong to the second of these 

 groups (that of the gray garnet schists) and his "Val Tremola 

 schists " to the third and fourth groups (green, black felspathic mica 

 schists and amphibolic rocks). On my geological map of the railway, 

 this whole series is marked by the figures IV. and V. and the sericite 

 schists of the Ursern valley are understood to be equivalent to the 

 gray mica schists of the south side. (Text to profile, French, p. 47 ; 

 German, p. 43.) 



In the composition of the gray mica schists, two species of mica, 

 at least, take part ; of these the gray is the characteristic one. In 

 the text to the profile (German, p. 45; French, p. 49) this is 

 described as "not positively identical with paragonite, though con- 

 taining soda, as shown by blow-pipe tests ; potash-mica also 

 occurring in the same schist. When fused to a yellowish white 

 enamel, it becomes intumescent giving a yellow tint to the flame. 

 It has a silky lustre and talcose appearance under the microscope ; 

 with a silver-white or grey colour, which often assumes a greenish 

 hue. In the immediate neighbourhood of quartz-veins with copper 

 pyrites, ironspar, cyanite, tourmaline, muscovite, calcspar, etc., this 

 same soda-mica often turns apple-green like that of -pregrattite ; this 

 green colour is of dubious origin (NiO, CuO, Cr-Q^?), and seems to 

 fade on long exposure to light. The blackish-tint and semi-metallic 

 lustre in the black garnet schists and in many calc schists is due to 



