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MESSRS. G. A. J. COLE AND J. W. GREGORY 



a bank of serpentine. Round the margins of this gabbro-mass, 

 except where it joins the serpentine, the texture is especially coarse, . 

 and there is not the slightest sign of a passage into the finer-grained 

 rocks of which the variolite is a selvage. 



The last outcrop of the gabbro is on the extreme east of the area, 

 where it occurs running from the bottom of the Gimont valley up 

 the east slope to the level of the platform, just north of Gr. Gimont. 

 On the lower part of the slope it abuts against the calc-schists that 

 form the floor of the valley : but the junctions higher up were not 

 exposed, and no special attention was devoted to this mass, as it 

 lies outside the area especially considered in this paper. The usual 

 diabase-dykes and veins of green felspar occur in it. 



Wherever the gabbro is exposed, it is found to be associated with 

 dark green or black " serpentines." On the south-west spur of 

 Le Chenaillet these occur in irregular patches in the gabbro, from 

 which, by the elimination of the felspar and decomposition of the 

 pyroxene, a gradual and complete passage to the '•' serpentinous" 

 masses can be traced. In the gabbro on the west flank of Le Che- 

 naillet true serpentine appears as a long dyke traversing the rock. In 

 the upper Chenaillet valley a similar serpentine runs from the end 

 of the gabbro on the north slope of the dry tarn-hollow across the 

 Col du Chenaillet, where it is covered by tuff (see sketch, fig. 2), 

 and here, as well as in the ridge that runs south, it is associated 

 with brecciated serpentine. On the west flank of Mt. La Plane, 

 that is, on the right slope of the lower part of the east Chenaillet 

 valley, the serpentine occurs as a dome-shaped mass, irregularly 

 underlying tuffs ; both the face of the serpentine and the layer of 

 vein-quartz that often marks the junction are greatly slickeusided, 

 and it appears as if the serpentine had been faulted up into the tuffs. 

 Pinally, looking east from Mt. La Plane across the Gimont valley, 

 one recognizes, by the striking slate-blae tint of the talus-slopes of 

 the upper part- of the opposite bank, a still larger development of 

 serpentine. From the north of Gr. Gimont it extends as a great 

 band along the side of the ridge, at about the 2200-metre contour, 

 for some distance to the south. This area is of interest, as it is that 

 which, by its size and development, is most akin to the serpentines 

 that crop out as elliptical patches throughout the range of the 

 Cottian Alps. 



It was doubtless on the black patches of "serpentinous " matter in 

 the south-west spur of Le Chenaillet that Elie de Beaumont based 

 his theory of the complete passage from the gabbro to the serpentine, 

 a theory which was accepted by Del esse * and Lory f, and reafiirraed 

 by Hebert in 1877 J. Microscopic examination of this " serpentine,'" 

 however, shows that two very different rocks have been confounded 

 together. The dark green masses of the spur of Le Chenaillet are 



* Delesse, Ann. des Mines, ser. 4, t. xvii. p. 130. 



t Lory, " Description geologique du Dauphine," para. 292, Bull. See. Stat. 

 Isere. t. vii. (18G4), p. 83. 



I Bull. Soc. geol. France, ser. 3, t. v. p. 266. 



