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



contains an abundance of slightly altered flakes of that mineral, as 

 in the Wildschonau serpentine discussed by Hatch * and Cathrein f. . 

 As, however, there is so little olivine in the gabbros and the diabases, 

 the serpentine must have been formed from great segregation-masses 

 of this mineral. The existence of such segregations in basic rocks 

 is well known from the descriptions of Prof. Juddj. But it is 

 possible that one of the dykes of serpentine in the gabbro on the 

 west flank of Le Chenaillet, containing as it does evidence of former 

 felspar, may be due to the intrusion of a gabbro richer in olivine than 

 is usually the case. 



At several points round the margins of the serpentine is found a 

 serpentine-breccia similar to that described by Prof. Bonney§. The 

 most striking mass is that on the east slope of the valley that runs 

 south from the Col du Chenaillet, while a little of it is well shown just 

 to the north of the Col. A similar bed occurs above the slicken- 

 sided serpentine on the east flank of the lower division of the eastern 

 Chenaillet valley. Examined under the microscope, the rock is seen 

 to consist of broken pieces of serpentine, the " Maschenstructur " 

 in which indicates its production from olivine. Secondary horn- 

 blende in the shape of long wisps, forming light-green dichroic 

 areas, indicates the former existence of pyroxene : flakes of altered 

 felspar also occur. These three constituents, and a multitude of 

 magnetite-granules, are scattered irregularly through a brown 

 matrix. In several specimens there are many small grains of calcite 

 and occasional lumps of limestone ; the latter appear in sections as 

 dusty areas traversed by veins of clear calcite with the usual irre- 

 gular outlines and polysynthetic twinning. This rock, resulting 

 from the breaking-up of serpentine with pyroxene and some felspar, 

 may be named a picrite-breccia. 



YI. The Dykes. 



Though earlier observers have accepted the gradual transition 

 from euphotide to the variolite-diabase, we have, as has already 

 been stated, found no sign of such a passage. On all the exposures 

 examined, the gabbro ends abruptly against the more compact rocks. 

 In all probability the gabbro was once a deep-seated mass that 

 consolidated at the base of the volcano ; but it now seems to form 

 a platform under the diabase, exposed where dome-shaped masses 

 have been pushed through the softer rocks, or where the valleys 

 have cut down to it. This view is supported by the abundance of 

 diabase-dykes traversing the gabbro in all its exposures. These 

 vary in width from half an inch or less to about 4 feet. Their course 



* F. H. Hatcli, " Uebei- den Gabbro aus der Wildschonau in Tirol und die 

 aus ilnn hervorgehenden scbiefrigen Gesteine," Tseb. Min. u. Pet. Mittb. Bd. vii. 

 (188(5), pp. 75-87. 



t A. Catbrein, " Ueber Wildscbonauer Gabbro," ibid. pp. 189-194. 



X J. W. Judd, "On tbe Tertiary and Older Peridotites of Scotland," Quart. 

 Journ. Geo]. Soc. vol. xli. (1885), p. 358. 



§ 'J'. G. Bonney, " Notes on some Ligurian and Tuscan Serpentines," Geol. 

 Mag. 1879, p. 3()5. 



