Vol. 52.] LUSTRES ' OF MONT JO VET (SAVOY). 9 



On the other hand, the member of the schist series which at first 

 sight appears least altered is the massive limestone of the Grande 

 Cote, nearly in the centre. Even between the least altered of the 

 limestones of the schist series and the Triassic dolomite there is a 

 great difference. Microscopic examination shows that the latter is 

 a normal dolomite, and does not contain any authigenons crystals 

 of albite. The limestones of the schist series, on the other hand, 

 are really calciphyres, show traces of intense crumpling, and con- 

 tain abundant crystals of plagioclase and epidote. If the schists be 

 really younger than the Trias, it appears inexplicable why these 

 should have been so intensely altered, while the dolomites in contact 

 with them should have escaped. 



It is therefore contended that the evidence is conclusive that the 

 Triassic dolomites have been deposited unconformably on the edges 

 of an older series of schists. It remains now to consider what 

 evidence is available to determine with which member of the schist 

 series in the Western Alps these must be correlated. The contorted, 

 lustrous, black, talcose schists which occur on the summit of Mont 

 Jovet remind one at once of the ' schistes lustres ' of the classical 

 area around Cesana, i. e. of the .' Kalkschiefer ' of Diener, the 

 ' Calcescisti ' of the Italian geologists, or the ' Upper Archaean 

 schists ' of Prof. Bonney. Zaccagna — whose acquaintance with 

 these schists is intimate — has no hesitation in regarding these as 

 part of this series. Lory, who knew both series extremely well, 

 also unhesitatingly identified the Mont Jovet rocks as 'schistes 

 lustres.' 



It must be admitted that in one respect these rocks are not typical 

 representatives of the ' schistes lustres ' series, and that is the great 

 development of massive limestones and quartzitic beds. Bands of 

 the same character as these occur in the ' schistes lustres ' around 

 Cesana ; but both the limestones and grits are there represented 

 by very thin layers and films, and the main mass of the formation 

 consists of black lustrous schists. 



It is these massive limestones which Bertrand has assigned to 

 the Lias, and macroscopic examination of a specimen does not show 

 anything fatal to this view. Microscopic study, however, reveals 

 the fact that the rocks are entirely crystalline, and have undergone 

 intense alteration. Sections of specimens from a boss in the upper 

 part of the valley of the Nant Gele, on the northern slope of the 

 Grande Cote, and from the summit of this peak, show that they 

 consist in the main of crystalline, poly synthetically twinned calcite. 

 A black carbonaceous dust occurs in irregularly crumpled and broken 

 lines through the rock. Some small grains of plagioclase (deter- 

 mined by Lory as albite) are scattered in the calcite at irregular 

 intervals. Grains of epidote are fairly numerous. The rock is 

 therefore a calciphyre, and is very different in structure from the 

 Jurassic limestones of the neighbourhood. 



The black lustrous schists, with which these limestones are seen 

 to be interstratifled both on the central peak of Mont Jovet and on 

 the flanks of the Grande Cote, are, however, thoroughly typical 



