UNDERGROUND WATERS AND MINERAL VEINS. 637 



actions; the cavities thus opened have served as channels for 

 waters which^ on their passage, have dissolved a part of the sub- 

 stance to deposit it afterward purified by crystallization — a fact 

 very much like what we habitually observe in our laboratories. 

 This mode of veined structure is most frequent in the limestones 

 of regions that have been dislocated. The Alps furnish many ex- 

 amples of it along escarpments of a considerable extent. 



Modifications have also been imposed upon the eruptive rocks, 

 under the influence of the waters that have traversed them ; but 

 they are of a different character from those that we have been 

 considering, not only on account of the heat that has prevailed 

 among them, but also on account of the composition of the rocks. 

 Various mineral species, grouped under the name of zeolitlis, may 

 be observed among the eruptive rocks, as crystals adorning innu- 

 merable cavities, such as we may see produced in existing volcanic 

 lavas, by the disengagement of the vapors which these lavas ex- 

 hale down to the moment when they are completely solidified. It 

 is easily seen that these zeolitlis were not formed at the same time 

 as the mother-rock, but after it had become consolidated and 

 turgid. They always assume the same disposition, whatever the 

 age of the rocks. Sometimes agate is associated with them, as at 

 Oberstein, in the Palatinate, where that stone was mined in an- 

 tiquity, and in Uruguay, where it is extracted at this time. Its 

 concentric zones, laid one upon another in successive moldings, 

 testify clearly to a gradual deposit, evidently of an aqueous nature. 

 Waters of incrustation are producing under our eyes deposits of 

 carbonate of lime of identical structure. The varied colorations 

 of the successive zones of agate which are utilized in the making 

 of cameos correspond with very slight variations in the nature of 

 the precipitant liquid. The limpid crystals of Iceland-spar, to 

 which physics is indebted, since Huygens, for most important 

 discoveries in double refraction and the polarization of light, are 

 associated with zeolitlis in the cavities of ancient lavas, and 

 originated at the same epoch. 



We do not any longer have to resort to erroneous or vague 

 conjectures to explain the origin of these minerals, for we have a 

 demonstration, that might be called experimental, which throws 

 the clearest light on all their details of it. The excavations made 

 at Plombi^res in 1851, to increase the flow of the springs, brought 

 to light in the deep trenches of the subsoil a part of the old Ro- 

 man underground conduits that had escaped the ravages of the 

 barbarians. They also disclosed a masonry-work of Mton and 

 bricks carefully built around the thermal springs, so as to isolate 

 them from the neighboring river and the gravel, in which they 

 were in danger of being scattered and cooled. Every spring im- 

 prisoned in this masonry, as it rose from its source, could only 



