332 ON THE VAKIOLITIC KOCKS OF MONT GENEVRE. 



EXPLANATION OF PLATE XIII. 



[The numerator of the fraction expressing the degree of enlargement of an 

 object represents the magnifying-power of the objective with which it was 

 viewed.] 



Fig. ] . Crystal of plagioclase in the great porphyritic diabase on ridge of Le 

 Chenaillet, traversed by a crack and subsequently restored by mate- 

 rial optically continuous with the original crystal, X V- 



2. Junction of diabase-dyke and gabbro, S.W. spur of Le Chenaillet. The 



diabase has a selvage of formerly glassy matter, green and somewhat 

 altered, and showing black globulitic aggregates, x V* 



3. Tuff" containing fragments of altered scoriaceous glass and variolitic 



rocks. Val du Gondran. X %^. 



4. Tuff containing fragments of greenish altered glass and of compacter 



lavas with radial groups of plagioclase. East ridge of Gimont 

 Yalley. x ^^ 



5. Porphyritic diabase of ridge of Le Chenaillet, some inches from its 



variolitic selvage, showing radial grouping of the plagioclase. X V. 



6. " Pseudocrystallites " and curving cracks intimately associated in large 



spherulite in the variolite. North end of ridge of Le Chenaillet. 

 XV. 



Discirssiojs". 



Prof. Bonnet had been over the Mont-Genevre, but had not reached 

 the exact area described by the Authors, though he had seen some of 

 the rocks mentioned. From his general knowledge of the district, 

 he appreciated how careful their description had been, and believed 

 with them that the variolites merely represented the pyromeride- 

 stage — a feature which, in his experience, was very rare amongst 

 the basic rocks. He stated that the Authors were correct in assign- 

 ing the gabbros and serpentines of the region to the ordinary Alpine 

 types, and was interested to learn that the variolite had nothing to 

 do with the gabbros. 



Prof. JuDB also congratulated the Authors on their thorough 

 treatment of an interesting subject. He wished to correct a 

 mistake of his own : judging from specimens of Hawaiian rocks 

 which he had seen, he was led to suppose that the tachylytes of 

 Kilauea occurred in extensive masses ; but Dana had shown that 

 the crusts of glass were never more than two inches thick. 



Prof. Blake compared the variolite of the Durance with a rock in 

 Anglesey occurring in a similar way. 



Mr. Gre&oet, in reply, said that though much of the old variolite 

 came from the north side of the ridge, nevertheless some of the rock 

 probably occurs in the valley to the south. It would be extremely 

 interesting to find a true British variolite in Anglesey. One of 

 the type-places in the Alps where there was supposed by E. de 

 Beaumont and others to be a passage from gabbro to serpentine 

 was this valley. The supposed serpentines which gave rise to this 

 view were not really examples of that rock. 



