O^r THK VARIOLHTC EOCKS OF MO^TT GENEVKE. 307 



may be irregular, as they may branch and enclose masses of the 

 gabbro, or they may form long straight regular dykes, often marked 

 on the denuded surface by a long gully in the more resisting 

 gabbro. 



The dykes are usually composed of a compact fine-grained rock, 

 of a dull green colour ; some, however, are more coarsely crystalline, 

 and the felspars are visible to the naked eye. Under the micro- 

 scope the coarse-grained dykes are shown to consist of lath-shaped 

 felspars ophitically included in a ground of secondary hornblende. 

 The felspar is turbid and worn, and the crystals are often surrounded 

 by a zone of fresh felspar, which has restored the original crystal- 

 line form. In many cases a fresh felspar-crystal has been deve- 

 loped, surrounding several turbid ones, which appear as if ophiti- 

 cally included in it. The titaniferous iron is passing into leucoxene, 

 the white bands of which are very marked. Olivine appears to have 

 been rarely present, and is indicated by dark brown decomposition- 

 products. We may therefore fairly call this rock a diabase, using 

 that ill-defined term in Hausmann's sense*, that is, for a labradorite- 

 pyroxene rock with green decomposition-products. A more finely 

 grained dyke traverses the gabbro on the slope below the west face 

 of Le Chenaillet, and in this the rock is less altered ; it is mainly 

 composed of a granular aggregate of felspar and pyroxene, with 

 green decomposition-patches formed from the latter ; titaniferous 

 iron occurs in scattered grains, and there is a little olivine. In 

 most of the dykes the rock is finer on the margins than in the 

 centre, and there is a thin devitrified glassy selvage. This is well 

 shown in a specimen collected low on the flank of the Col du Gon- 

 dran ; the junction is irregular, thin tongues of the diabase pene- 

 trating the gabbro, while broken fragments of the latter are enclosed 

 in the diabase. The rock of which the dyke is composed is a finely 

 crystalline mass of acicular felspars in a basis coloured green by the 

 predominance of the decomposition-products. Towards the margin 

 of the dyke the texture becomes finer, till at the edge it passes into 

 a glass, full of cumulitcs and some incomplete crystals of felspar. 

 Epidote-veins traverse the diabase in all directions near the margin 

 (PI. XIII. fig. 2). The dykes are usually not spherulitic, the only aj)- 

 proach to this structure being the formation of the cumulites above 

 referred to. Eut in one case a huge block of compact diabase has 

 a distinct variolitic selvage ; along the margin of the diabase there 

 are many amygdules, some of which are elongated. Though this 

 specimen was not found actually in situ, there can be little doubt from 

 its position that it belongs to the dykes intrusive in the gabbro. 



The rarity of the variolitic selvage in the diabase-dykes is remark- 

 able, since it is so constant an accompaniment of the great diabase- 

 masses about to be described. Since, moreover, the dykes are 

 not seen to run from the gabbro into the upper diabase-series, 

 they may possibly not be connected with the latter. The spherulitic 

 and glassy selvages arc, however, evidence that they are not mere 



* J. F. L. Ilausmann, ' Ueber die Bildung des llarzeebirges,' Giittinseu : 

 1842. 



