390 PEOr. T. G. IiO:N'XEY ox THE SO-CALLED ' GNEIS> 



25. On the so-called ' Gxeiss ' of Caeboxiferofs age at Guttannex 

 (Canton Berne, Switzerland). By Prof. T. G. Boxxey, D.Sc, 

 LL.D., F.E.S., Y.P.G.S. (Read May 11th, 1892.) 



As a text precedes a sermon, so two qiiotatioDS may fitly begin 

 this communication. The first comes from a letter written by 

 Dr. Heim, and printed in this Journal ^ as a part of the discussion 

 on my paper entitled ' On the Crystalline Schists and their relation 

 to the Mesozoic Eoeks in the Lepontine Alps ' : — '-'- In the Cential 

 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 dis- 

 tinguished. Such rocks are phylliies, chlorite-scJiists, felsite-schists, 

 mica- schists, and especially sericite-gneisses, all of which we regard 

 with certainty as Palaeozoic. The proofs are the following: . . . Traces 

 of fossils have been often found (trunks of Calamites from Gut- 

 tannen, in the Haslithal. &g.) . . . The Palceozoic foYmsitions mostly 

 show an intimate tectonic relation to the crystalline schists, and have 

 been converted petrographically into crystalline schists." The 

 second quotation is from the reply which I was permitted to append 

 to the above letter : — " Some of these [Carboniferous rocks] I have 

 examined, and think I knoAV them well enough to demur to Dr. 

 Heim's statements concerning them. I have seen, in the Berne 

 Museum, the specimen with ' the Calamite-like stem.' When this 

 rock is proved to be a gneiss I shall be prepared to consider the 

 propriety of extending this name to the Gres Feldspathiqiie of Nor- 

 mandy, or that of mica-schist to some rocks of Carboniferous age at 

 Yernayaz, in Canton Valais, or of calling the Torridon Sandstone of 

 Scotland a granite.'' 



Erom these quotations it is obvious that to some extent the question 

 on which we are at issue is one of nomenclature. By gneisses and 

 schists - I mean rocks which have assumed a crystalline condition in 

 situ; which, if originally clastic, have been so completely changed by 

 molecular re-arrangement that their tbrmer constitution is a result 

 of inductive inference, and is not immediately revealed by minute 

 study of the structure of the specimen.'' Suppose a painstaking 

 workman had so ingeniously fitted together quartz, felspar, and 

 mica as to deceive the eye even when a lens was used, this would 

 not justify us in calling the mosaic a granite. It would be only a 



1 Vol. xlvi. (1890) p. 236. 



^ I always use the term ' schist ' to mean a foliated rock— the only legiti- 

 mate sense. See Jukes, ' Student's Manual of Geology,' p. 142 (3rd ed., bv 

 Sir A. Geikie). 



^ As we now know, many schists and some gneisses are the result of the 

 crushing, followed by mineral cliange, of rocks that probably, and in some 

 cases certainly, are of igneous origin. Some gneisses and schists also may be 

 simply peculiar conditions of igneous rocks. These, however, I exclude from 

 the present discussion because they have no direct bearing on the question at 

 issue. 



