Popular Science Monthly/Volume 17/June 1880/Studies in Experimental Geology
|STUDIES IN EXPERIMENTAL GEOLOGY.|
M. A. DAUBRÉE, in his recently published "Études Synthétiques de Géologie Expérimentale" (Synthetic Studies in Experimental Geology), besides giving the results of studies on the formation of meteorites and on the production of the chemical and physical phenomena of geology, describes a series of interesting and instructive experiments upon the manner in which the mechanical operations of which the earth's strata bear witness were brought about.
M. Daubrée began this series of experiments with an investigation of the processes by which the convolutions of the stratified beds were produced. With a very simple apparatus, a frame of iron strengthened by long screws crossing two of its contiguous sides, he subjected flexible laminæ, as representing in miniature the strata of the globe, to pressures of varying energy and direction. Under these constantly measuredFig. 1. — Double system of conjugate fractures developed in a plate of glass, GG, by the effect of the torsion produced by the turning-handle, TT. EE, block of wood in which the other end of the glass plate is fastened.(One sixth the natural size.) efforts he saw reproduced all the characteristic traits of the geology of regions whose strata are warped, synclinal valleys, anticlinal crests, slopes, and reversals of strata, strata in the form of C, strata in the form of S, etc.
Prosecuting his experiments beyond the limits of the elasticity of bent laminæ, M. Daubrée went into the investigation of the origin and mode of formation of terrestrial fractures. He was able by means of his new processes to produce imitations of joints and faults in all their details. An unforeseen result of his experiments was, that they disclosed evidences of mutual coordination between natural fractures which observation alone had failed to notice. This point deserves a careful examination. A rectangular plate of glass, GG (Fig. 1), was clamped between two jaws of wood tightly bound by screws so as to form a kind of vice, EE. The other end of the plate was fitted to a wrench, TT. By turning the wrench around horizontally, M. Daubrée twisted the plate of glass so that it speedily broke into a thousand pieces. Having previously taken care to paste a sheet of 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 70º 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 in Fig. 3: the fossil has been broken up, and its pieces have been more or less separated from each other, exactly as in the natural examples.
By pursuing a similar line of research, M. Daubrée has succeeded in imitating a characteristic feature of the structure of large chains of
mountains, which Saussure observed on Mont Blanc. The masonry-work of Mont Blanc, says this author, is divided into great leaves having their planes exactly parallel to each other, and parallel to the direction of the chain. He further satisfied himself that the leaves,
nearly vertical in the center of the mass, assumed inclined positions in their lateral parts, and dipped symmetrically toward the central axis, so as to present in their transverse section the form of a half-opened fan. Little Mont Blancs can be reproduced in miniature, with a structure like that described by Saussure, in this way: Take clay which has been previously well mixed and nearly dried, and cut into the form of a square prism; and, having put it between two square plates of the same dimensions as the base of the prism, subject it to the action of the hydraulic press. In the operation a beard (bavure) or overflow runs out from each of the four-lateral faces, the expanding form of which, in consequence of the change of pressure, adjusts itself with the faces of the prism. The deformed mass exhibits on a transverse fractureFig. 4.—Production of the fan-shaped structure in a mass of clay forced to flow out from between two parallel plates. The horizontal pressure of the plates makes it take on first a leafy structure, then the fan-shaped structure as soon as it has passed beyond the limits of the plates. (One third the natural size.) an essentially schistous texture, which is thus disposed: in all of the part inclosed between the plates, the leaves are nearly parallel to the two walls, but in the part which passes beyond the plates the leaves bend over and diverge from the axis so as to be parallel to the two exterior surfaces, while they themselves separate more and more. The leanness is especially marked near the two external surfaces; it is generally much less so toward the central part. This experiment furnishes a facsimile (Fig. 4) of the leafy structure called fan-shaped.
It remains to call attention to the consideration of the mechanical actions developed in the crust of the globe as the source of the heat-movements to which the metamorphism of rocks is due. M. Daubrée has been led by his experiments to conclude that such is the origin of this phenomenon. The mechanical action which is required to make rocks schistous is enough to heat them to a considerable degree. It is known that a very slight elevation of temperature is sufficient to produce chemical reactions in the depths of rocky masses. The quarry-water, with which all the rocks are impregnated, and that which finds its way to them through fissures, could give rise to reactions which might be prolonged for a protracted period. La Nature.