254 Dr. Irving — On the Airolo Schists. 



formulation of his ideas on this subject in this letter shows that he 

 has seen no reason to depart in any essential matters from the 

 diagnostic contrast, which he drew some twelve years ago, between 

 the rocks of the Central Massif (viewed as a great crystalline com- 

 plex) and the rocks of the flanking ranges of the Alpine Chain. 1 

 Von Hauer (as representing the Austrian geologists) and Credner 

 (as representing those of Germany) may be cited in favour of the 

 same view. So that one is at a loss to understand what people 

 can mean, when they attempt to depreciate the results of the micro- 

 scopic and field work of such a worker as Prof. Bonney by charging 

 him with " holding exceptional views," 3 in maintaining the existence 

 of a great Archaean complex with an individuality (so far as the 

 broad general facies are concerned) altogether its own, in contrast 

 with any great complexus of rocks of later age. Why even Heim does 

 not venture to speak of the Palaeozoic rocks of the Todi-Windgallen 

 Group (as a series) as anything more than ' half-crystalline ' ; 3 yet 

 this is perhaps the most powerfully compressed and contorted region 

 of the whole Alpine system. Eliminate the local and exceptional 

 structural features which are met with, resulting in some cases from 

 contact-metamorphism, sometimes from excessive pressure and shear- 

 ing [e.g. in the Todi-Windgallen Group), at others from the detrital 

 material derived from the central crystalline massif being exception- 

 ally siliceous and crystalline, — and you get a great sedimentary 

 series flanking the whole Alpine Chain quite distinct from the 

 Central Massif. This seems to be Prof. Bonney's view; and this 

 the foremost of living Swiss geologists has declared to be the view 

 still held by him and his confreres in the year 1890. 4 And so the 

 authors of this flank-attack upon Dr. Bonnej^'s work seem to have 

 impaled themselves upon the horns of a pretty dilemma! This is 

 certainly rather ' sensational ' for them. 



Now it must be distinctly borne in mind that Heim's descriptions 

 of the rocks of the Central Alps are almost entirely based on 

 macroscopic examinations and field observations ; and he himself 

 emphasizes the desirability of a detailed (durchgehend) microscopic 

 examination. 5 This it is which forms the chief strength of Prof. 

 Bonney's position. For the Swiss geologists are rather late in the 

 field with this branch of research ; and, while they have been lagging 

 behind, other cases of alleged "reduction of rocks of later date to 

 the condition of crystalline schists," 6 have been exposed and 

 condemned on a closer examination with the microscope. 7 



The late Prof. R. D. Irving 8 defined a typical crystalline schist 

 as " a rock of completely crystalline interlocked texture, which is 



1 Op. cit. Bd. ii. p. 40. See also Chem. and Physical Studies, pp. 89, 124. 



2 Q.J.G.S. February, 1889, p. 109. 3 Op. cit. Bd. i. pp. 41-52. 



4 From a conversation which the present writer had with Dr. Heim over his 

 sections in September, 1888, it has seemed all the way through this controversy that 

 the real difference between Heim and Bonney has been much exaggerated. 



5 Op. cit. Bd. i. p. 43. 6 Q.J.G.S. loc. cit. 



7 See {e.g.) ' A supposed case of metamorphism of an Alpine rock of Carboniferous 

 age,' Geol. Mag. Decade II. Vol. X. pp. 507-511. 



6 Etudes sur les Schistes Crystalline, p. 93. 



