278 THE WALDENSIAN GNEISSES DISCUSSION. [May 1 894, 



difficulties of mapping. There was some evidence of Tertiary (or 

 Upper Mesozoic) volcanic activity farther west, though faulted 

 junctions prevented clear demonstration in the most desirable case. 

 There, in the Mont Genevre and Chaberton region, the presence of 

 Mesozoic sedimentaries enabled one to immediately recognize folds, 

 faults, and thrusts, the importance of which might easily be under- 

 estimated in the area of crystalline schists and gneisses. 



Mr. Vaijghan Jennings and Dr. G. J. Hinde also spoke. 



The Author, in reply, admitted the need for caution and careful 

 mapping. He did not think the inclusions could be segregations, 

 owing to their difference in composition from any of the minerals 

 of the gneiss and their resemblance to those of the altered ' pietre 

 verdi ' ; segregations do occur, and are easily distinguished. No 

 aplite-dykes occur in the gneiss, and the aplite is mineralogically 

 almost identical with the selvages of the gneiss masses ; the evidence 

 of the Crissolo section seems conclusive that the aplite-dykes are 

 offshoots from the gneiss. As regards the argument from the 

 unaltered nature of the gneiss, it was admitted that rocks in mass 

 could readily escape ; but this could hardly be the ease with dykes 

 ranging from 100 feet to 1 foot in width. The absence of pebbles in 

 the conglomerate tells also very strongly in favour of the recent age 

 of the gneiss. The interl ami nation of thin beds of the radiolarian 

 phthanites with the calc-schists renders it very difficult to explain 

 their occurrence by any system of infolds. He had no doubt of the 

 occurrence of igneous rocks of post-Palseozoic age in the Cottians. 



