Page:Popular Science Monthly Volume 17.djvu/213

Rh paper around the glass so as to make the fragments keep their places and preserve their relative distribution to each other, he found that the fractures, instead of assuming a uniform direction, formed in the glass a network of geometrical regularity. They seemed to be grouped in two directions or systems, equally inclined to the axis of torsion. The two conjugate systems of fractures generally crossed each other at very open angles, the measure of which depended on the relative dimensions of the two sides of the plate. Sometimes the angle was a right angle, sometimes it was reduced to an angle of 70° or less.

These artificial fractures present close analogies with certain geological characteristics of different regions. An example of localities in which a correlation is presented between the subterranean fractures and the reliefs of the surface is presented in the cretaceous beds of the south of France. If we examine attentively a well-made map of this district, we shall see that from the principal valleys, which are parallel to each other, branch out a large number of other valleys, likewise rectilinear and parallel with each other. We can see in them how the thin pellicle which we call the crust of the earth has yielded to strains or torsions analogous to those which the wrench has impressed upon the plate of glass, and how it has become fissured in directions coordinate to each other. In the Spanish part of the massive Mont Perdu, the cretaceous and nummiliferous rocks, in the main horizontal, have been raised to a height of nearly ten thousand feet, and are notched to the depth of four or five thousand feet by narrow valleys, the walls of which are nearly vertical. Another example of this kind of reticulated system is presented in the forms of the coasts, fiords, and principal valleys of a part of Norway.

We already know that the schistous or leafy structure presented by many tracts called metamorphic must be attributed to real laminations. M. Daubrée has make experimental studies on the distortions which the forms of the fossils in the schistous rocks have undergone. The trilobites and mollusks of the neighborhood of Angers very rarely present themselves in any other than deformed shapes which seem like caricatures of the animals from which they are derived. These deformities can be perfectly imitated in experiments. If we inclose the shell of a crawfish in a mass of lead which we then cause to pass through a flattening-mill, we can inflict upon the crustacean a malformation quite like that of the silurian trilobites. A remarkable exemplification of the changes of form which fossils contained in rocks that have become schistous have undergone is presented by the belemnites of different localities in the Alps, in cases where they have been broken into pieces and their segments have been more or less removed from each other. M. Daubrée has produced similar forms of breakage by laminating blocks of lead in the interior of which belemnites had been previously inclosed. Fig. 2 represents a belemnite thus inclosed in a block of lead of which only a half is shown. The effect of laminating is shown