Vol. 54.] SCHISTS OF THE ST. GOTHARD PASS. -^63 



diameter. The largest are sometimes associated with veins ; 

 the smaller are generally scattered throughout the rock. 

 (4) Pressure evidently has acted on the rocks, but to a variable extent. 

 Sometimes its effects are inconspicuous (unless we regard the 

 actinolitic habit of the hornblende as an indication) ; sometimes 

 the rock is almost fissile. Traces, however, of a cleavage- 

 foliation can be generally detected, and are often obvious. 



Fig. 4. — Diagrammatic sketch of a face of garnet-actinolite roch, 

 {Slopes above Airolo.) 



[Some of the actinolite-crystals are quite 4 inches long.] 



(5) Pressure, though it may account for this foliation, seems in- 



adequate as an explanation of the more marked instances of 

 mineral banding, in which felspar, mica, or hornblende may 

 predominate for a thickness of ^ inch upwards. 



(6) The relation of these varieties in the field appeared explicable 



on either of two hypotheses : (a) that a group of sedimentary 



rocks, which varied somewhat in chemical composition, had 



undergone extreme metamorphism ; or (6) that the apparent 



bedding had been produced by fluxional movements in a magma, 



which, either from differentiation or from the intrusion of one 



variety into another, was not uniform in composition.^ 



There is much to be said in favour of both these hypotheses. 



In my earlier work, at a time when the effects of pressure on rocks of 



this kind were ill understood, and those of fluxional movements 



were almost unknown, I adopted the former one ; but I am now 



convinced (especially since my work last summer) that the second 



hypothesis affords a better explanation of the phenomena as a whole. 



^ I do not remember to have found the above-described group of rocks so 

 well developed in any other part of the Alps, although gneisses of the 

 St. Gothard type — that named Montalban by the late Dr. Sterry Hunt — 

 occur sometimes, and seemingly at about the same horizon. These ' Tremola 

 Schists,' however, appear to be uncommon, though I have seen rather similar 

 rocks in the Upper Iselthal and the Zillerthal, and specimens from other 

 Tyrolese localities in the Museum at Innsbruck. Prof. Lap worth has shown 

 me some fine specimens of these rocks which he obtained at Kongsvold 

 (Norway). 



