Page:Popular Science Monthly Volume 34.djvu/527

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quently, the molluscan fossils called belemnites have been broken up and had their segments more or less scattered.

Since the schistose structure has been found to be independent of the stratification, the cause of a geometrical disposition so remarkable and so general has become the subject of various hypotheses. It has been successively attributed to electric effects, to terrestrial magnetism, to the heat of the globe, and to a beginning of crystallization. Exact observations, however, teach us that the cleavage of stratified beds is related on the one hand to the actions which have deformed the fossils in the same strata, and on the other hand to the axes of the warping and the great lines of dislocation. The phenomenon should most probably be attributed to mechanical action.

The demonstration has been confirmed by some very simple experiments. Clay under compression assumes a leafy texture; but, for this, it must have a certain degree of plasticity. If too dry, it crumbles; if too wet, the laminæ, while they are formed, are not separable. I have got more decisive results from forcing clay to flow, in a jet, under hydraulic pressure. In this case, very well defined leaflets are produced, and that upon bands of several metres, in the direction of the pressure and the movement. All of these artificially laminated pastes resemble natural schistose rocks in their fracture. In these various flows of the plastic mass, the neighboring particles do not advance uniformly. The differences in the velocities which they acquire cause them to slide upon one another; and the schistose texture, the direct consequence of this sliding, is, we may readily conceive, necessarily determined in reference to the direction of the flow. The deformations of fossils and the drawing out of belemnites have been reproduced in this way, and thus experimentally explained.

We shall now consider how the fundamental facts of metamorphism imply the necessary action of subterranean waters. The mineralogical modifications peculiar to the phenomena have incontestably taken place at a higher temperature than now prevails on the surface of the globe. We base this conclusion upon the analogies of these beds with the eruptive rocks, and especially upon the presence of numerous anhydrous silicates, which form one of their most remarkable features. The proper heat of the globe decreasing from the deeper parts toward the surface, the sediments deposited in the ocean, at the relatively low temperature that reigns there, should, when they have been covered by other strata, acquire a higher temperature by reason of their greater distance from the radiating surface. The superposition of masses as heavy as are those of some of the stratified beds has often been enough to determine, after their deposition, a considerable heating up of the lower masses, especially at periods when