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Page:Popular Science Monthly Volume 34.djvu/525

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Stratified rocks have sometimes acquired these characters in the vicinity of eruptive rocks. In several localities of the Tyrol, the Triassic limestone in contact with melaphyre has been transformed into white marble for a thickness of more than five hundred metres, while pyroxene, spinel, tourmaline, and other crystalline minerals have been developed at the same time.

Clay schists have suffered mineralogical transformations in proximity with granitic eruptions. Even half a century ago, De Boblaye pointed out the presence, in Brittany, of fossil shells among the schistose rocks, which also contained, in testimony of the heat to which they had been subjected, large crystals of silicious minerals, as of andalusite or made, and staurotide. The groupings of the latter species in the form of a cross have been long remarked, and have caused the name of croisette to be given to it. These remarkable modifications of the schists, which constitute a sort of radiation around the granitic flows, extend to distances varying from a few hundred metres to three kilometres. The heat to which the strata have been subjected by the intrusion of the eruptive mass is undoubtedly one of the causes of it; but the watery emanations which accompanied the eruption of the granite, and which are revealed to us by inclusions in the mass, attest that water has played a no less important part in it.

There is, however, something still more remarkable than this in the phenomena of metamorphism. Sedimentary rocks, occupying whole regions, bear evidence of profound modifications, without its being possible to discover the slightest eruptive cropping out. One of the most common examples of this phenomenon is that in which clay rocks have become phyllads. The rocks of that name, although they consist essentially, like the clays, of silicates of alumina, differ from them in their cohesion. They refuse to mix with water. The strata of the Ardennes, the Taunus, and other regions of western Europe, in which this mineralogical condition was first verified, belong to the most ancient geological epochs; and from that fact this crystalline texture was for a long time regarded as exclusively appertaining to sedimentary deposits of a very remote age. Hence the name of transition beds which was given them. It was thought that in the sea in which these matters were deposited, following the primitive or crystalline beds, there continued to operate a chemical precipitation of silicates which were mingled with arenaceous and calcareous deposits. It was subsequently recognized that this half-crystalline condition resulted from a transformation posterior to sedimentation.

The opinion that the mineralogical condition of these beds is not a necessary consequence of their antiquity, receives confirmation from the fact that formations in other countries, also belong-