336 THE GEOLOGY OF THE ALPS. [XIV. 



very conspicuous along the southeast side of Mont Blanc, ex- 

 tending into the Valais, and is regarded by Lory as a peculiar 

 modification of the trias and lias, so enormously thickened and 

 so profoundly altered as to be very unlike these formations to 

 the northwest of Mont Blanc. In this view he is followed by 

 Favre ( 666, 753). The serpentines and related rocks of 

 this series are by De Beaumont, Sismonda, and Lory considered 

 to be eruptive. The latter speaks of these as eruptions con- 

 temporaneous with the deposition of the strata, probably ac- 

 companied by emanations which effected the alteration of the 

 sediments. According to Favre, they are clearly interstratified 

 with the lustrous argillo-talcose schists, micaceous limestones and 

 quartzites of the great series, and are by him placed in the trias. 

 He has particularly described those of Mont Joret and those of 

 the Yal de Bruglie", near the Petit St. Bernard, where they are 

 immediately interstratified with greenish schists, and associated 

 with steatite, hornblendic and gneissic strata. The serpentines 

 of Taninge in the Chablais, to the northwest of Mont Blanc, 

 he also classes with these in the trias. The conclusions of Lory 

 and Favre as to the geological age of these crystalline schists 

 and limestones appear to us untenable in the light of Sismon- 

 da's investigations. If we admit with the latter that the whole 

 section of the tunnel represents an uninverted series, and ^-ith 

 Favre that its uppermost and uncrystalline portion at Modane 

 is truly of carboniferous age, it is clear that the great mass of 

 crystalline schists which underlie the latter should correspond 

 more or less completely to the pre-carboniferous crystalline strata 

 to the northwest of Mont Blanc. Among these latter, in fact, 

 as observed by Favre, there occur at Col Joli and Taninge crys- 

 talline limestones and talcose schists like those of Maurienne. 

 According to this view, which harmonizes the conflicting opin- 

 ions, and makes the crystalline schists and limestones of the 

 southeast pre-carboniferous, the anhydrites, with limestones, 

 talcose slates, and quartzites seen in the Mont Cenis tunnel, are 

 not the equivalents of the gypsum and cargneule of the trias, 

 l>ut may correspond to the anhydrites which, with gypsum, 

 dolonnt*'. serpentine and chloritic slate, are met with in tho 

 primitive schists of Fahlun in Sweden. 



