Page:Popular Science Monthly Volume 17.djvu/215

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which, in consequence of the change of pressure, adjusts itself with the faces of the prism. The deformed mass exhibits on a transverse fracturePSM V17 D215 Demonstration of geological pressure plates using clay.jpgFig. 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.


THE theory now to be considered was first advanced by Mr. McLennan, in his work entitled "Primitive Marriage." It will be found stated in the following words in his "Studies in Ancient History,"[1] a reprint, with additions, of the former work:

"We believe this restriction on marriage (i. e., exogamy) to be connected with the practice in early times of female infanticide, which,

  1. "Studies in Ancient History," comprising a reprint of "Primitive Marriage." By John Ferguson McLennan, M.A., LL.D. Quaritch, London, 1876.