iil2 PflOE. T. G. BONNEY ON CRYSTALLINE SCHISTS AND THEIR 



with that of the cr3'stalline schists, quartz, rutile, &c. So if we pay iio 

 attention to the latter constituents, and claim the underlying rock 

 and the included fragments as representatives of the Carboniferous 

 series, then, if the dark-mica schists of the Binnenthal are to be 

 regarded as Jurassic, how is it that these have been so completely 

 altered, while a neighbouring rock, very similar in composition, 

 which before Triassic times had attained to practically its present 

 state of metamorphism, has been so much less changed* ? 



Again, some 25 miles away to the south-west near the side of 

 the Gorner glacier and on the south flank of the Hochthiiligrat, 

 in a region consisting partly of calc-schist, marble, quartz-, chlorite-, 

 and other schists t which lithologically resemble the most highly 

 crystalline members of the Upper group of Alpine schists, partly of 

 gneisses of the ordinary character, which appear to underlie the 

 latter, we find a small mass of rauchwacke. It is the usual yellow, 

 friable, non-metamorphic rock. At the first sight it appears to bo 

 interbedded on the mountain-side with the above-named schists and 

 gneiss, but a closer examination indicates that it occurs in a very 

 " patchy " way, and that its outcrop hardly coincides with that of 

 the other masses. This rauchwacke contains not only scales of 

 white mica, very detrital in aspect, but also indubitable fragments 

 of the silvery gneiss with which it is apparently interstratified. Even 

 if it be contended that these fragments only represent the gneissic 

 series, the greater age of which is admitted, we have still to explain 

 the anomaly of a rock which retains the characters of an ordinary 

 sedimentary deposit, directly underlying a great mass (containing 

 rocks of very different composition, but some of which are also lime- 

 stones) which has become highly crystalline. 



In short, this rauchwacke, wherever I have seen it J, is not a rock 

 to which I should apply the term metamorphic. I have little doubt 

 that it will be found, in many places, to contain fragments of the 

 great group of schists to which the Piora schists unquestionably 

 belong. But I always felt so convinced that it was a rock of com- 

 paratively late date, and that its apparent inters tratiiication with 

 the schists or gneisses was illusory, that until my last visit to the 

 Alps I never took the trouble of searching for fragments. 



It might, however,^ be suggested that in the above cases I have 

 mistaken a consolidated talus of schist and rauchwacke — a formation 

 of recent date — for an integral part of the latter rock. It may 



* Oil referring to the ' Beitrage zur geol. Karte der Schweiz,' Lief, xxiii. 

 pi. i., I find this objection is not likely to be advanced. The rauchwacke is 

 represented as underlying all these dark rocks of the Binnenthal, which are 

 marked as forming one mass, and thus are implicitly claimed as Jurassic. 



t These, in the new edition of the Zermatt sheet of the Survey map, are 

 coloured " Lias- Jura," while the older map simply indicated tlie lithological 

 character of the deposits. This is one of tlie unfortunate changes which Jias 

 converted the map from an accurate petrographical record into an expi-ession 

 of the hypothetical opinions of certain geologists, and thus has greatly dimin- 

 ished its value. 



I I have often come across it, and have also examined typical specimens in 

 the Berne Museum. 



