Geological Society of London. 139 



Discussion. 



Dr. Geikie having sent an abstract of the paper by Professor 

 Bonney, read to the British Association at Newcastle, to Prof. Heim, 

 the latter had favoured him with a resume of his views on the 

 subject, of which the following is a translation : — 



"It appears to me that Professor Bonney starts from a misunderstanding. It 

 has often been maintained that Mesozoic rocks can become crystalline ; but no 

 Swiss geologist bas, so far as I know, ever asserted that the crystalline schists of 

 the Central massif of the Alps are metamorphic Mesozoic rocks. In the printed 

 abstract of his British Association paper, however, which you have sent to me, 

 Professor Bonney attacks this supposed assertion — one that has never been made. 



" I am in perfect accordance with those who know the Central massif best 

 (Baltzer, Fellenberg, etc.) in fixing the following points: — 



" 1. Crystalline schistose rocks of Mesozoic age exist at Scopi, in the Valserthal 

 (Graubiinden), in the Userenthal, on Piora, at the Nufenerpass, in the Val Canaria, 

 in the Ganterthal and numerous other places. Such rocks are : — ■ 



" (a) Clay-slates, with mica, garnets, zoisite, staurolite, rutile, and Belemuites, 

 the latter being crystalline and granular. 



" (b) Clay-slates, with mica, staurolite, etc., and garnet, alternating with the 

 Belemnite-schists. 



" (c) Green plagioclase-amphibole schists, alternating with the Belemnite-schists. 



" (d) Micaceous phyllites and calcareous mica-schists. 



" (ei Marble with mica, which has undergone linear stretching, going over into 

 ' Malm-kalk ' with crinoids. 



" We have never given the name ' crystalline-schists ' to these rocks, nor have we 

 ever regarded them as such, but always as sedimentary metamorphosed zones 

 (synclinal basins) between the central massifs. Professor Bonney is right in saying 

 that they have not the aspect of true crystalline schists. It is true there are some 

 varieties which it would be difficult to distinguish in the hand-specimen, and without 

 stratigraphical evidence, from true crystalline schists. Stratigraphically, they always 

 present themselves as ' Mulden-zoues ' accompanied by other sedimentary rocks. 



" In the Ventral massifs occur rocks which exactly resemble true crystalline schists 

 in mode of occurrence. Petrographically, they are related to them by passage-rocks ; 

 at least, the line of separation is not easily distinguished. Such rocks are phyllites, 

 ehlorile-schisis, fel site- schists, mica-scbists, and especially sericite-g)ieissis, all of 

 which we regard with certainty as palaeozoic. The proofs are the following : — 



" («) In some places in these zones are found intercalated beds of graphitic 

 and sometimes even anthracitic schists (Brisenstock, etc.). 



" (b) Traces of fossils have been often found (trunks of Calamites from Guttanen 

 in the rJaslithal, Carboniferous plants in strips wedged in on the TSdi, etc). 



" (c) At the end of the Central-massif distinct zones of Carboniferous slates 

 are often developed out of the zones of these sericitic gneisses ; and the 

 synclinal ('Mulden') nature of these zones, in comparison with the old 

 granitic gneisses, is shown there by the wedging in of still younger 

 unaltered sedimentary rocks. 



" (d) I have already shown in my Todi-Windgallen group that even the 

 Verrocano (Permian), when nipped in between crystalline schists, assumes 

 a close resemblance to them, and appears as a part of the crystalline 

 Central-massif. 



" Fragments of these rocks are found in the Triassic rauchwacke. but this is not 

 the case with the garnetiferous schists of Scopi which are younger than the Rauch- 

 wacke, and belong to the true sedimentary synclinal zones (' Mulden'). 



" A great unconformity exists in the Central Alps between Palaeozoic and Mesozoic 

 formations, but not between Palaeozoic and Azoic. 



" I. The Palaeozoic formations mostly show an intimate tectonic relation to the 

 crystalline schists, and have been converted petrographically into crystalline schists. 

 The central-massifs consist, perhaps to the extent of two-thirds, of true old crystal- 

 line schists, older than the Cambrian, in part, perhaps, the primitive crust ( Krstar- 

 rungskruste, granite-gneisses, protogine) ; and to the extent of about one-third 

 of Palaeozoic mica-schists, sericite-schists, amphibolites and other similar rocks which 

 have been derived by dynamic metamorphism from Palaeozoic slates, sandstones, and 



