Page:Popular Science Monthly Volume 33.djvu/655

Rh actions; the cavities thus opened have served as channels for waters which, on their passage, have dissolved a part of the substance 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 examples 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 zeoliths, may be observed among the eruptive rocks, as crystals adorning innumerable cavities, such as we may see produced in existing volcanic lavas, by the disengagement of the vapors which these lavas exhale down to the moment when they are completely solidified. It is easily seen that these zeoliths 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 antiquity, 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 zeoliths 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 Plombières in 1851, to increase the flow of the springs, brought to light in the deep trenches of the subsoil a part of the old Roman underground conduits that had escaped the ravages of the barbarians. They also disclosed a masonry-work of bêton 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 imprisoned in this masonry, as it rose from its source, could only