ON THE VARIOLITIC ROCKS OF MONT GENEVRE. 325 



Turning to the compacter series, the abundant dykes show that 

 considerable activity was going- on during the formation of the 

 variolitic lavas. In the field we pictured to ourselves a huge 

 volcanic cauldron, its centre being most probably among the great 

 intrusive masses of Mt. La Plane ; in this basin, excavated in the 

 calcareous schists, the viscid lavas would heave and seethe upon one 

 another, the surfaces of junction between them becoming coated 

 with spherulitic glass, the product of their more rapid cooling. A 

 crust would form from time to time across the crater, to be torn 

 through again by occasional explosive action. Hence agglomerates 

 would be formed intimately connected with the lavas, and spherulitic 

 glassy fragments would be abundantly thrown up, together with 

 globular masses of compactor rocks. As the andesitic lavas became 

 piled thickly upon one another, the new material would burst up 

 through rocks already solid, and would form the more crystalline 

 dykes of the Chenaillet ridge and Mt. La Plane. But even these 

 would prove their affinity to the earlier lavas by consolidating 

 against them with a vesicular and spherulitic selvage. 



Indeed, it is this abundance of basic glass, now devitrified and 

 furnishing the " variolite,'* that makes the area of Mt. Genevre 

 remarkable among volcanos old or new. While it is probable that 

 many such centres may be recognized in Piedmont and the northern 

 Apennines, similar occurrences are rare in other parts of Europe ; 

 and we believe that the best modern analogue is to be found in the 

 great craters of Hawaii. Although we do not pretend that the 

 compact andesites of Mt. Genevre possessed the marvellous fluidity 

 of the lavas of Kilauea, yet they must have borne, when fresh, 

 a remarkable resemblance to the well-known palioelioe of the 

 Sandwich Islands. The engraving in Dutton's ' Hawaiian Vol- 

 canoes '*, and the exquisite photographic view of the lava-floor of 

 Kilauea which accompanies a recent article by Prof. J. D. Danaf, 

 restore for us, as it were, the surface of our *' variolite-diabases ; '*' 

 while the vertical joint-faces in the foregrounds of these illustrations 

 hint at structures precisely similar to those seen on the arete of 

 Le Chenaillet. Prof. Dana's articles, indeed, considerably strengthen 

 the impression made upon us in the field. At Kilauea the lavas 

 have a glassy crust, which is often scoriaceous, and half an inch to 

 two inches thick. " The crust is a crrt^(?r-feature," writes Prof. 

 ])anat, " for I have not seen it on the lavas outside. . . .The lavas 

 exuded through the crust from the liquid mass below, above alluded to 

 as making seams, streamlets, and knobby surfaces, are covered some- 

 times with separable scoriaceous glassy crust, though commonly 

 having a solid glassy exterior half an inch or so thick." At Mt. 

 Genevre, even allowing for the crushing and obliteration of delicate 

 scoria) during earth-movements, the more solid glassy selvages 

 appear to have been everywhere predominant. But the scoriaceous 

 character of the diabases, and even of the edges of the massive 

 dykes, bears ample witness to the vajjours that escaped throughout 



* U.S. Geol. Survey, 4th Annual Report, publ. 1884, p. 98. 

 t Amer. Journ. Sci. vol. xxxiv. (1887) pi. iv. p. 3()4. 

 \ Ibid. pp. 354 and 355. 



