ON THE VARIOLITIC ROCKS OF MONT GENEVRE. 313 



the Mt. Gcnevre area the spherulitic selvage is found on every 

 conspicuous surface of division throughout the rock, and often seems 

 to have become infolded when still viscid by the pressure of adjacent 

 masses (fig. 5). Often a subsidiary jointing has been set up, as 

 is common in spheroids of contraction *, columns being produced 

 which radiate from the centre, and produce a tesselated effect on 

 the outside. This feature is best seen, however, in the globular 

 masses found in the rocks styled hy us variolitic tuffs. 



Where very thin, the variolitic crust is liable to become lost by 

 exfoliation and decomposition ; and its outermost layer is commonly 

 soft, and coated with dark-green alteration-products. Bat in the 

 majority of instances the spherulites are easily recognizable, and 

 hundreds of specimens might be collected in an hour which would 

 show the passage from compact grey diabase to typical " variolite of 

 the Durance." 



The microscope shows us that the green matrix of the variolite 

 becomes less and less in quantity as we proceed from the surface 

 towards the centre of the spheroidal masses of diabase. The spheru- 

 lites come into contact with one another, as in the similar " apha- 

 nite " of Liguria t ; but even at some distance from the selvage they 

 are still differentiated. In the interior of the rock their place 

 is taken by beautiful stellar or brush-like groups of plagioclase, the 

 rays of which are straight. These rays contain dark axes, occupying 

 about a third of their bulk, formed of included or imperfectly crys- 

 tallized material ; they often bifurcate at the ends, branches are set 

 on at intervals, and forms resembling skeleton-crystals are thus 

 built up, though the individual little rods composing them have 

 different optical orientations (compare PI. XIII. fig. 5). 



The spherules of the variolite itself are often of a translucent 

 brown colour, the more characteristic grey appearance being due 

 to alteration. The rays, moreover, of the brown examples are 

 pleochroic, as in ordinary tachylytes. .The matrix becomes also 

 browner in the interior of the spheroidal masses of the rock, resem- 

 bling thus the residual glass in the great porphyrite dykes. 



Porphyritic crystals occur occasionally in the variolite. We have 

 not detected olivine, but may record felspar, magnetite, iron-pyrites, 

 and transparent little pscudomorphs after pyroxene. Not unfre- 

 qucntly, embryo prisms of felspar, with characteristic bifurcated 

 ends, appear in the midst of the spherulites, as if developed at an 

 earlier stage. We must mention also the " pseudocrystallites " so 

 clearly distinguished by M. Levy X, which occur in so many of the 

 larger globules, and which wo are inclined to regard as little fissures 

 due to fracture or contraction. Much as they often resemble the 

 constituents of a crystalline meshwork, there is evidence in many 

 of our examples of a tendency to branch and become irregular. We 

 do not find that the rays of the spherulites, as seen in polarized 

 light, run on without interruption through these lighter areas; on 



* See, for example, Serope, 'Considerations on Volcano.s,' 1825, p. 140. 



t Mazzuoli and Issol, Boll. R. Coniit. geol. d'ltalia, vol. xii. (1881), p. 330. 



j Bull. Soc. geol. France, 3" ser. t. v. (1877), p. 238. 



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