Page:Popular Science Monthly Volume 34.djvu/526

This page has been proofread, but needs to be validated.

ing to the most ancient systems, do not participate in these crystalline characters, but in their argillaceous rocks are similar to those which are found in recent formations. This is the case in Sweden, Russia, the United States, and Canada. But it has been observed in those places that the strata have not been strongly dislocated as in the regions we have just been speaking of, but have retained their original horizontality. To this circumstance they doubtless owe their preservation. The mineralogical contrast between formations of the same age corresponds, therefore, with an essential difference in their bearing.

There are countries where formations of less antiquity have also suffered profound transformations. The Alps afford fundamental data on this subject. In the face of the rocks of different ages—Carboniferous, Triassic, and Tertiary—which enter into their composition, one is surprised at the special physiognomy which each one of them presents as compared with what we observe in beds of the same age in other regions, where they have remained horizontal. A general influence has, therefore, acted upon a part of the vast region of the Alps. It has affected rocks of every epoch, even those of the Lower Tertiary—that is, a series of beds many thousand metres thick—and that, although eruptive rocks are very rare in it.

With the mineralogical changes which we have just noticed is associated a modification of texture that depends on the same cause. It is well known in the slates as the schistose or laminated structure. The fissile rocks which it characterizes have the property of detaching themselves in thin plates—that is, of cleaving in certain directions. Observations made in various countries have demonstrated the important fact that the planes of cleavage are quite distinct from the planes of stratification. Instead of being parallel to the layers, they are frequently oblique, and—what is still more conclusive—while the planes of stratification have been bent and exhibit a variety of inclinations, the planes of cleavage pursue a regular direction, regardless of the most pronounced inflections, and remain constantly parallel to one another. This independence shows, besides, that the planes of cleavage were produced, not only after the beds in which they are manifest were deposited, but also after they had lost their primary horizontality. The schistose disposition, very frequent in the most ancient fossiliferous rocks, sometimes persists in more recent formations, when they have been subjected to energetic dislocations. In many localities of the Alps slates are quarried into the Tertiary formation.

An important characteristic of the schistose rocks is the considerable deformations which the fossils in them have received, as is seen in the trilobites of the Angers slates. Not less fre-