The Variolitic Rocks of Mont Genèvre

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Abstract
1. I ntroduction . Though the series of structures found in the acid igneous rocks has now an almost complete parallel in the basic group, a few gaps in the latter still remain unfilled. The spherulitic and perlitic structures of ordinary rhyolites are now well known in the basic lavas ; but the only rock that seems homologous with the pyromerides of Jersey and North Wales is that known as the Variolite of the Durance, which has been described as the selvage of a compact euphotide. Impressed by the perlitic and other structures of this rock, one of us suggested in 1888 that it was a devitrified tachy lyre; while a few weeks later, the other attempted to settle the question of its true nature by an examination of the rock in the field. As a series of heavy snowstorms prevented anything being done on that occasion, we visited Mont Genèvre together last August, to examine the igneous mass to the south of the pass, which had been mapped by Lory as an intrusive euphotide, and from which it was probable that the variolites of both the Durance and the Dora had been derived. II. P revious W ork O n T he V ariolite O f T he D urance . Variolite is a rock that has been of interest since the time of the Romans on account of its supposed medicinal value; its European sources, however, were lost till it was found near Lucca, early in the 17th century; previously the supply had been drawn from America, where it

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