358 PKOP. T. G. BONNET ON THE GARNET-ACTINOLITE [Aug. 1 898, 



geol. Karte der Schweiz,' 1873, Lief, xv, pp. 65, 66). In the 'Livret- 

 Guide Geologique,' Yl^me Congr. Geol. Int. 1894, they are mentioned 

 at p. 156, and in the section are coloured as Biindner Schiefer (Ob. 

 Trias bis Dogger), an identification which, it is needless to say, is 

 more easily assumed than proved. Prof. C. Schmidt refers to them 

 in a discussion of the geology of the Simplon region (Arch, des Sci. 

 Phjs. et Nat. 1895, p. 84), and regards them with the adjacent 

 crystalline rocks as ' schistes cristallins anciens ' and ' roches arche- 

 ennes' (p. 85). See also Eclog. Geol. Helvet. vol. iv (1896) p. 367, etc. 

 In Mitth. der naturforsch. Gesellsch. in Bern (1895, Nos. 1373-1398, 

 p. 73) they are also briefly noticed, and in a section drawn across the 

 Ofenhorn schists corresponding with the ' Upper Schists ' of the 

 Yal Piora district, are coloured as ' Jiingere metamorphe krystall. 

 Schiefer, Mesozoisch bis Cambrisch.' A wide Kmit of choice ! 



II. Desceiption of Sections. 



Time will be saved by describing as concisely as possible the 

 more important facts noted in the field, indicating the conclusions 

 to which they point, and by discussing afterwards the evidence 

 obtained from microscopic examination. I will take the localities 

 studied from west to east, commencing with the St. Gothard ; for 

 in walking beyond it up the Val Bedretto I did not diverge from 

 the lower part to investigate these schists, though from what I saw 

 I infer that they soon become much less conspicuous than on the 

 St. Gothard Pass. 



{a) The St. Gothard.^ 



By following the general line of the high road we reach, perhaps 

 about 550 feet above Airolo, a well-foliated greyish biotite-gneiss 

 in which red garnets are scattered sporadically, at first small, 

 then of larger size, often with a slight streak-like association. 

 A little higher up these occasionally range from ^ to | inch 

 in diameter. Here the rock shows a banded structure : zones, 

 perhaps 8 or 9 inches thick, alternating with others from 10 to 

 12 inches, in which garnets are less abundant. Then hornblende 

 (generally actinolitic) becomes more abundant. For instance, at 

 about 650 feet up, a small quarry exhibits a grey garnetiferous 

 schist, poor in hornblende, passing down into a darker variety, 

 with fewer garnets, but with the usual large and rather blade-shaped 

 actinolites. This mineral now becomes more frequent, and the 

 garnet- actinolite schists^ continue tiU the road passes from the 

 left to the right bank of the torrent descending the Yal Tremola. 

 In fact the whole ascent seems to be over one or other of these 

 rocks, with an occasional ' relapse ' into biotite-gneiss ^ and one or 

 two greener bands. 



^ I had been ab"eady four or five times over this section, and we visited it in 

 1897, both going to and coming from Val Piora. 



^ I use the term in a general sense, for variations are not unfrequeut, and 

 the two minerals seem to occur often in inverse proportion. 



' As at about 1400 feet above Airolo. 



