ON THE YARfOLITIC EOCKS OF MONT GENEVRE. 319 



bronzite being entirely absent from the tuff. The microscope easily 

 distinguishes between the particles of serpentine, with their veins of 

 magnetite, in the breccia, and the fragments of diabase in the green 

 and metamorphosed tuff. 



It is remarkable, indeed, that the serpentine should have thus 

 yielded to earth-movement without a real intermingling of the rocks 

 at contact. We presume that, at the period of brecciation, the 

 variolite-tuff had become as tough and hard as its unweathered 

 masses are to-day. 



The great locality for the tuff is Le Chenaillet itself; and on the 

 western slopes the characteristic mode of weathering is admirably 

 displayed. The roughly piled masses are divided, not by stratifi- 

 cation, but by vertical joints, and form, under denuding influences, 

 great dyke-like spurs running out towards the valley at right angles 

 to the ridge (fig. 6). As is common in ancient volcanic agglome- 

 rates, there is also a marked tendency to the formation of piunacles 

 and spires. 



YIII. Relations of the Hocks to one another and to the 

 Stratigraphical Series. 



To summarize, then, the relations of these igneous rocks to one 

 another, we may safely assert that the gabbros and serpentines, so 

 intimately connected, form a floor or undulating platform on which 

 the variolitic series has been piled. The gabbro is exposed at four 

 distinct points : — above the forks of the Durance, at a height of 

 2250 metres ; on the south-west angle of Le Chenaillet, at 2450 

 metres ; in the floor of the upper Chenaillet valley (2350 to 2550 

 metres) ; and on the east flank of the Gimont valley, among the 

 limestones, as low down as 2100 metres. Serpentine occurs inde- 

 pendently at 2200 metres on the west slope of Mt. La Plane and 

 along the ridge north of Cima Saurel, some 200 metres higher. 



While, then, the true euphotide seems to have at the surface little 

 of the importance assigned to it by earlier observers, it none the less 

 forms a base to the eruptive series and is probably of similar age. 

 But the original junctions of the highly crystalline rocks with the 

 diabase-series, or the passages from one type to the other, have been 

 lost during subsequent earth -movements, the gabbro being often most 

 coarsely developed at its present margin, and exhibiting there the 

 eye-structure of gabbro-gneiss. These facts, and the abundant 

 slickensides in the gabbro, show that the upper series may have been 

 shifted over the hard crystalline masses, leaving us in doubt as to 

 their contemporaneous origin and connexion. 



The gabbros, however, were broken through in places by eruptive 

 rocks, which foim numerous dark dykes and veins. We arc led to 

 regard these as representing either the last upwclling of molten 

 material through the cracks of the consolidating gabbro, or as the 

 lines of fissure through which the variolitic rocks attained the surface. 



We have, however, been unable to detect the passage of these 

 dj-kes, so numerous on the west flank of Le Chenaillet, into the 



