Vol. 54.] GARNET-ACTINOLITE SCHISTS OP THE ST. GOTHAED. 357 



27. The Garnet- AcTiNOLiTE Schists on the Southeen Side of the 

 St. Gothaed Pass. By T. G. Bonnet, D.Sc, LL.D., F.R.S., 

 Y.P.G.S., Professor of Geology in University College, London, 

 and Fellow of St. John's College, Cambridge. (Read May 18th, 

 1898.) 



Contents. 



Page 



I. Introduction 357 



II. Description of Sections 358 



(a) The St. Gothard. 

 (6) Val Canaria. 

 (c) Yal Piora. 



III. Results of Microscopic Examin ation 364 



lY. Inferences as to Mineral and other Changes 368 



V. Connexion of Changes with Earth-movements 371 



I. Inteobuction. 



At intervals during the last 20 years I have studied the well- 

 known garnet-actinolite schists which are exposed on the southern 

 slope of the St. Gothard Pass from perhaps 500 to about 1600 feet 

 above the mouth of the tunnel.^ They extend westward along the 

 flank of the Val Bedretto to beyond AU'acqua, with one slight 

 interruption near this spot, and can be followed eastward to the 

 vicinity of the Pizzo Columbe, that is, for a total distance of from 

 16 to 17 miles, the breadth of the outcrop, which is locally inter- 

 rupted by elongated masses of more normal hornblendic schist, 

 being at its maximum about 1| mile. They are intersected by 

 the Yal Tremola ^ and the Val Canaria with its tributary glens, and 

 are exposed on the right bank of the Yal Piora. Here the lower 

 part of the mass must be about 6700 feet above sea-level, that is to 

 say, the outcrop rises at least 2500 feet in about 5 miles. 



My later visits threw some light on the origin of these rocks ; 

 but I felt that more work in the field was necessary before I could 

 come to any conclusion. Therefore I determined to return to the 

 district and examine more particularly the outcrops near the Yal 

 Piora, of which hitherto I had seen little, having spent my time on 

 another group of schists, which apparently overlie them. This 

 intention was carried out in July 1897, and I had the good fortune 

 to be accompanied by my friend and former pupil Mr. John 

 Parkinson, P.G.S., to whom I am indebted for much assistance both 

 then and afterwards. 



So far as I have discovered, with the kind help of Dr. J. W. 

 Gregory, no one has paid much attention to this group of rocks. 

 They are described briefly and clearly by K. von Fritsch (' Beitrage zur 



1 These figures are only approximate. I have measured (by aneroid) the 

 lowest outcrop by the high road, but there are others yet lower in the slopes 

 some distance to the east. Moreover, it is not easy to fix precisely the exact 

 position of the upper limit — nor does it seem to me very important. 



^ For which reason I have referred to them since 1886 (Pres. Addr. Quart. 

 Journ. Geol. Soc. vol. xlii, Proc. p. 72) as the Tremola Schists. 



