320 MESSES. G. A. J. COLE AND J. W. GKEGORY 



upper diabase series. Where the variolitic tuffs are in direct 

 contact with the gabbro, such dykes ought to form prominent 

 objects. The great porphyritic diabase, however, that crosses the 

 ridge of Le Chenaillet is certainly intrusive in the tuffs, and pro- 

 bably has similar relations to the gabbro ; but the gabbro does not 

 reappear to the north of it, and thus even here the evidence is 

 inconclusive. There are abundant dykes of diabase in the variolitic 

 lavas and tuffs, and their characters link them closely with those 

 traversing the gabbro. Yet we find no evidence of passage between 

 the gabbro itself and this compacter group. An observer intent on 

 establishing the sedimentary or metamorphic origin of euphotide and 

 serpentine might, indeed, accept the eruptive character of the diabases 

 without prejudice to his views of the more crystalline series. 



The age of the variolitic group and associated gabbros has been 

 generally accepted, on the authority of Lory*, as later than the 

 Infra-Lias. AVe have observed something like contact-alteration 

 where the variolitic diabase abuts on limestone north of the fork of 

 the stream in the Chenaillet valley. Here, and also against the 

 gabbro in the Gimont valley, the limestone is broken and traversed 

 by abundant calcite-veins ; but this brecciated condition of the rock 

 precludes accurate determination of the existence of contact-meta- 

 morphism. Gastaldif has, indeed, denied such alteration at the 

 junction of the " pietre verdi " and the overlying limestone. Simi- 

 larly, the interesting limestone-fragments in the eruptive rocks afc 

 the Col du Chenaillet occur, not in the massive serpentine, but in 

 the brecciated variety : and it may therefore be urged that they 

 have become included as the result of subterranean crushing. AVe 

 think, however, that their distinct removal from the limestone of the 

 lower valley is fair evidence that they were carried up during the 

 intrusion of the igneous mass. 



But doubts now arise as to the real age of the stratified rocks 

 regarded by Lory as Liassic. The obscure fossils picked up on the 

 talus of Mt. Chaberton appear to have formed the basis of this con- 

 clusion t. Gastaldi and his collaborators, who believed in 1876 that 

 they had found evidence here of Cambrian and Silurian strata, 

 abandoned this view later, on the determination of their specimens 

 as Mesozoic. But Gastaldi claims to have proved the existence of 

 the Trias within the limits of Lory's " Calcaire du Brianconuais"§ ; 

 while it is clear that he would sweep the whole of the euphotide and 

 diabase group down into the " pietre verdi," and thus relegate them 

 to the pre-Pala30zoic. That this view was influenced by the gene- 

 ralization he had adopted with regard to serpentines in the abstract 

 may be seen by his refusal to admit the Eocene age of any of these 

 rocks in Tuscany \\. 



* "Descript. geol. du Dauphine," paragraph 290, Bull. Soc. Stat. Isere, 

 2= ser. t. vii. p. 79. 



t Boll. R. Comit. geol. cVItalia, vol. vi. (1875) pp. 352-3. 



+ Bull. Soc. geol. France. 2"= ser. t. xviii. (1861) p. 770. 



§ " Sui rilevainenti geologici fatti nelle Alpi piemontesi, 1877," Mem. R. 

 Accad. (lei Lincei, Rome, ser. 3. vol. ii. 1878, p. 959. 



II Letter in 1878 to S terry Hunt, Geol. Mag. 1887, p. 53G. 



