UNDERGROUND WATERS. 509 



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 trans- 

 formed into white marble for a thickness of more than five hun- 

 dred metres, while pyroxene, spinel, tourmaline, and other crys- 

 talline minerals have been developed at the same time. 



Clay schists have sufl:ered 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 con- 

 stitute a sort of radiation around the granitic flows, extend to dis- 

 tances 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, occu- 

 pying whole regions, bear evidence of profound modifications, 

 without its being possible to discover the slightest eruptive crop- 

 X)ing out. One of the most common examples of this phenome- 

 non 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 Tau- 

 nus, and other regions of western EurojDe, in which this miner- 

 alogical 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 sediment- 

 ary deposits of a very remote age. Hence the name of transi- 

 tion 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 precipita- 

 tion of silicates which were mingled with arenaceous and calca- 

 reous 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 confirma- 

 tion from the fact that formations in other countries, also belong- 



