ON THE TAKIOLITIC KOCKS OF MONT GENEVRE. 317 



clingue with variolitic rocks recorded by Morozzo * from near Susa 

 was a rock similar to those of Mt. Genevre. We fail to see, how- 

 ever, that Geinitz t has established satisfactorily that the schistose 

 variolite described by him from the Col de Sestrieres is a tuff and 

 not merely a metamorphosed portion of the diabase. 



We have not mapped t]\e boundaries of these fragmental rocks on 

 account of their intimate association with the variolite-diabase. 

 They may, indeed, be friction-breccias, or lavas broken up while 

 \iscid, or volcanic tuffs. Careful consideration inclines us to the 

 last-named of these views, and we regard them, as will be seen in a 

 later section, as the products of local explosive action. Against the 

 view that they are friction-breccias, we would urge that slickensides 

 are rare in their material, though common enough in the brecciated 

 serpentines ; that the dykes which must be practically contempora- 

 neous with them have not suffered from any similar crushing ; and 

 that we have found no admixture of gabbro, serpentine, or limestone 

 in these diabase-" breccias," even when they are hundreds of metres 

 in thickness. The smaller particles appear, moreover, to represent 

 the glassy types rather than the compactor diabase-lavas ; while 

 spherical " bombs ' are abundant, which remain unbroken and are 

 beautifully coated with uninjured variolite. Nor, on the other hand, 

 do we think that the characters of the matrix in these fragmental 

 rocks, when little altered, can support the suggestion that they are 

 lavas which became brecciated during flow. In such a case at least 

 some of the instances examined should betray, by spherulites or other 

 signs of imperfect crystallization, the former molten nature of the 

 ground-mass. The slaty bed against which the great dyke of Mt. 

 La Plane has cooled is also some evidence of the existence of ashes 

 and tuffs among the lavas at the time of their formation. 



A striking feature of these fragmental deposits is, as we have 

 hinted, the abundance of globose masses of compact diabase, their 

 surfaces being thickly coated with variolite. Although only slightly 

 vesicular, these appear to be of the nature of volcanic bombs. The 

 smaller ones are spherulitic throughout, and in almost all cases the 

 old glassy crust is exceedingly well marked. Some of these sphe- 

 roidal specimens, with their concentric coats, resemble the well-known 

 globes from the pitchstone of the Chiaja di Luna of Ponza. The 

 largest that we have measured was on the western slope of Le 

 Chenaillet, and was partly broken, showing within the characteristic 

 radiating columnar structure. The dimensions of this imperfect 

 mass were 70 centim. by 43 centim. by 45 centim. We believe 

 that the large and beautiful specim(3ns of variolite, spherulitic from 

 one side to the other, which are obtainable in the upper Durance 

 are residues from the breaking-up of the volcanic agglomerate rather 

 than from the massive diabase-lavas. The blocks thrown out into 

 the tuffs would naturally be at times completely vitreous. 



The smaller fragments are often scoriaceous and very angular, 

 and consist of variolite and more ordinary diabase in about equal 



* M6m. Acad, royale des Sciences, Turin, t. v. (1793), p. 169. 

 t Tscherm. Min. u. petr. Mitth. 1878, pp. 146-152. 



