310 MESSRS. G. A. J. COLE AND J. W. GREGORr 



pseuclomorphs after aiigite. Some very similar bodies are, however, 

 probably amygdules, filled with clear crystals, the single cleavage of. 

 which is suggestive of epidote. The alteration of the rock is 

 marked by the development of epidote in the felspars, and of chlo- 

 ritic and epidotic areas, and minute grains of epidote, in the ground- 

 mass. There is very little of the original glassy matrix preserved. 

 The rock was, however, originally an augite-andesite. In most 

 cases the only signs of radial structure are seen in the chloritic areas, 

 but in some specimens long rays of felspar are grouped about a 

 centre ; by a gradual increase in this structure a passage from the 

 ordinary andesite to a variolite-selvage can be traced (PI. XIII. 

 fig. 5). In other cases thfe selvage is full of porphyritic crystals 

 which seem to have floated to the margin. 



Another coarse porphyritic diabase-dyke, grey in colour, occurs 

 crossing the south-west spur of Le Cheuaillet in the diabase-tuff 

 above the junction with the gabbro. 



The last type of dyke is the dolerite that occurs in a mass on the 

 south side of the summit of Le Chenaillet, on the side of the valley 

 running to the Col du Chenaillet. It is a rather coarse-grained 

 rock, and we considered it in the field as more allied to gabbro. Its 

 microscopic structure, however, shows that it would be most appro- 

 priately named a dolerite ; the texture is ophitic, and the augite is 

 not schillerized. Its constituents are augite and plagioclase, with 

 some green decomposition-products and scattered crystals of titanic 

 iron and pyrites. The augite is in fairly large crystals, and, in con- 

 trast to the felspar, is remarkably fresh ; it is, however, much 

 cracked, and along these fissures decomposition has commenced in 

 places. In some cases a complete passage can be traced from fresh 

 augite at one end of a crystal into green decomposition-products at 

 the other, while in the middle a zone of diallage has been caused by 

 schillerization. Whether all the green areas are derived from 

 augite is doubtful, since in some cases, the junction is sharp between 

 a fresh augite crystal and the " viridite." The latter may have been 

 formed from accessory bronzite. The green areas appear, however, 

 to be mainly composed of actinolite, and a crystal of secondary 

 magnetite often forms a nucleus from which radiates a group of 

 actinolite-needles. 



YII. The Yariolitic Diabases and Tuees. 



We may now proceed from these dyke-rocks, on which variolite 

 occurs in situ as a selvage, to the mass of the compactor diabases 

 through which they were intruded. In 1861 * Lory reports that, 

 starting from near Le Chenaillet, " en avancant de quelques pas 

 vers Test, on trouve Veuplwtide bien caracterisee, a elements nette- 

 ment separes, qui continue de la jusqu'aux sommites et dans tout 

 le vallon affluent de la Poire." The euphotide proper is certainly 

 visible Irom this point, but occupies, as has been described, only a 



* Bull. Soc. geoL France, 2*= ser. t. xviii. (1861), p. 782. " Descript. gcol. 

 du Dauphine " paragraph 293. 



