Dr. Irving — On the Airolo Schists. 255 



possessed of a schistose parting due to a parallel or foliated arrange- 

 ment of the mineral ingredients, or of aggregations of those in- 

 gredients." This is about as satisfactory a definition as has yet 

 been given. And he complains ' that after the name ' metamorphisin ' 

 was applied to undoubted masses of a clastic origin which had 

 acquired a certain amount of schistosity (" a vague naming of an 

 unexplained process rather than an explanation"), it was loosely 

 and generally carried over to all classes of crystalline schists, whether 

 anything was known about their original condition or not. 2 "No 

 serious effort was made to trace them into unquestionable sedimenta- 

 ries, and when the attempt was made later it was found impossible 

 to do so." When then we begin to discuss a 'crystalline schist,' we 

 are treading on slippery ground, unless we agree on some definition 

 beforehand. We cannot tolerate the construction of a definition to 

 suit the exigencies of a special case ; yet it is not too much to say 

 that Mr. Teall was caught in this fallacy in the discussion of Prof. 

 Bonney's paper, at the Geological Society, on 22nd January, 1890. 3 

 He enumerated certain characters, which the altered Belemnite- 

 bearing rocks of the Nufenen Pass exhibit under the microscope; 

 but they do not satisfy the requirements of such a definition of a 

 crystalline schist as has been quoted above. At the same time he 

 frankly admitted that he had found on examination that '•' the garnet- 

 schists are quite different from the Belemnite-rock." No one can 

 therefore doubt that Prof. Bonney was standing on very firm ground 

 when he " gave a complete contradiction to any statement that these 

 fossil-bearing rocks are in any proper sense crystalline schists," 

 and asserted that the similarity between these and certain undoubted 

 crystalline schists of Val Canaria and Val Piora was only superficial, 

 as "under the microscope the differences were at once visible," 

 the two series of rocks having a totally distinct facies. All the 

 details of the evidence on which these statements are based will 

 appear when Dr. Bonney's paper is published. 4 



If we may judge from one of the first instalments of detailed 

 work in microscopic petrography which the younger school of Swiss 

 geologists have produced, we have certainly in the recent monograph 

 of Dr. Grubenmann, of Frauenfeld, 5 the earnest and the promise of 

 a rich harvest in microscopic petrography in the near future. There 

 is a thoroughness about Grubeninann's descriptive work, from his 

 combining the threefold method of field-observation, microscopic 

 examination, and chemical analysis, which is most satisfactory. The 

 pity is that we cannot say as much for the logical soundness of his 

 conclusions, and their bearing upon questions of general theory. 



1 Op. cit. pp. 94, 95. 



8 At a previous meeting of the Society the present writer entered his protest 

 against this assumption (Q.J.G.S., August, 1889, p. 503). 



3 See " Abstract of Proceedings of the Geological Society," No. 549. 



4 That paper having been now (May 19) published, this is found to be so. 



5 " Uebcr die Gesteine der sedimentiiren Mulde von Airolo,'''' by Dr. Ulrich 

 Grubenmann (Frauenfeld, J. Huber, 1888). I have to thank Dr. Heim for drawing 

 my attention to this, and Dr. Grubenmann for his prompt courtesy in sending me a 

 copy ; and I offer beforehand most ample apologies to the latter, if anything in this 

 paper may appear to him at all discourteous (etwas unhoflich). 



