GEOLOGY OF THE WESTERN ALPS. 163 



accuracy of details, but that the main sequence in the area 

 occurs in the order in which Zaccagna shows it we do not 

 feel inclined to doubt. On the main question as to the age 

 of the "schistes lustres," or "calcescisti" as he calls them, his 

 opinion is very decided. He divides the Archean into 

 three zones : the lowest or central gneisses, the middle or 

 mica-schists, and the upper or calc- schists. The two 

 latter were referred by Lory and Favre to the Trias, but 

 Zaccagna is positive that they cannot be referred to any 

 part even of the Palaeozoic. He says : " It is not therefore 

 possible to admit the idea of referring the calc-schists to 

 the Palaeozoic, more or less ancient, as there is a tendency 

 to do among some modern authors, Carez and Vasseur, 

 Kilian and Termier ". He maintains that the calc-schists 

 are so intimately associated with the mica-schists, and the 

 series of " pietre verdi " or "greenstones" which occur in 

 them, that they cannot be separated. He says, therefore, 

 that if the calc-schists are assigned to the Palaeozoic, so 

 also must be " all the other rocks which extend, alter- 

 nating with the calc-schists, even to the contact with the 

 central orneiss ". 



That two authors, studying the same country, traversing 

 the same sections, and often going together over critical 

 areas, could have arrived at such fundamentally different 

 conclusions is rather surprising. Fortunately, they both of 

 them rest their arguments on the same sections, so that the 

 ground is prepared for an independent survey. Thus 

 Mount Jovet, in the valley-of the Isere, has been selected 

 by both authors as a test case. According to Zaccagna, it 

 is an island of Upper Archean schists rising up through 

 the Triassic rocks, which were deposited against its flanks ; 

 while, as we have previously seen, Bertrand regards these 

 schists as Liassic limestones resting on a fan-shaped fold of 

 Triassic and Carboniferous rocks, though he admits (6, p. 

 99) that " ce froissement extreme des assises horizontales 

 du sommet de l'eventail est assez difficile a expliquer". It 

 is difficult to compare Zaccagna's sketch of the structure of 

 Mount Jovet (pp. 229-232) with that of Bertrand without 



becoming prejudiced in favour of the former. 



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