Vol. 50.] GNEISSES IK THE COTTIA.N SEQUENCE. 267 



TIL The Age op the "VValdensian Gneisses. 



In Part IV. of the present paper the evidence has been sum- 

 marized to show that, whatever may be the age of the gneiss, it is 

 younger and not older than the surrounding schists. If, therefore, 

 we wish to fix a minimum age for the rock, it is necessary first to 

 determine the age of the schists. This question has been twice 

 discussed in recent years by Prof. Bonney and Signor Zaccagna, 

 who both agree with Gastaldi, Baretti, and other Italian writers ; to 

 these must be added the name of Prof. F. Sacco, 1 who, in a 

 recent paper, accepts the Laurentian and Huronian correlation of 

 the beds. 



Elie de Beaumont and Fournet held that the schists were 

 Jurassic, but their arguments were not in accord with modern 

 methods ; their view has, however, been again expressed recently 

 by Prof. Stanislas Meunier 2 in a map in his ' Geologie Pegionale 

 de Prance.' Lory, on the other hand, assigned the ' schistes 

 lustres' to the Trias; this he taught in 1861, and reaffirmed 

 in his latest utterances. 3 He originally assigned 4 this age only 

 to the calc-schists that form the upper part of the series, but 

 ultimately found it impossible to separate these from the lower 

 mica-schists, and thus carried his line of Trias as far east as the 

 junction with the Waldensian gneisses near Susa. This view was 

 based on his mistaken identification of the whole of the ' Calcaire 

 du Brianconnais ' as Liassic ; the lowest cargneules and dolomites 

 of these are now universally admitted on palseontologieal grounds 

 to be Triassic. 5 The schists below them are, therefore, either 

 Palaeozoic or pre-Palaeozoic. Lory would no doubt have accepted 

 this change ; he would have argued that all the evidence which 

 made him originally assign the ' schistes lustres ' to the system 

 immediately preceding the limestone series was quite unaltered, 

 and that this simply involved calling the schists Carboniferous or 

 Permo-Carboniferous instead of Trias. This would, of course, be 

 a mere matter of detail, and would not affect the principles involved 

 in the discussion. 



It therefore becomes necessary to consider what are the oldest 

 unaltered fossiliferous rocks in the district, and what are their 

 relations to the Cottian schists. 



Along the eastern side of the Cottians the oldest fossiliferous beds 

 are the Pliocenes, along the extreme edge of the crystalline area ; 



1 ' L'Age des Formations opkiolitiques recentes,' Bull. Soc. Beige Greol. Pal. 

 vol. v. (1891) Mem. pp. 60-95. 



2 Paris, 1889, p. 430. 



3 ' Sur la Constitution et la Structure des Massifs de Schistes cristallins des 

 Alpes occidentales,' Congree Greol. Internat., 4 me Sess. Londres, 1888, Compte- 

 rendu (1891), pp. 86-103. 



* Bull. Soc. Stat. Isere, ser. 2, vol. vii. (1864) p. 94 (§ 298) ; ibid. vol. v. (1861) 

 p. 88 (§43). 



See Diener, ' Gebirgsbau der Westalpen,' pp. 18, 19. 



t2 



