Among instances of the bodily deformation of rocks by lateral pressure, the case of the phyllade aimantifère of Monthermé is well known. By its strong cleavage this rock gives evidence of considerable lateral compression. Professor Renard has shown that prior to this compression the magnetite crystals already existed in the rock, and were surrounded by a coating of chlorite. The crystals yielded to the pressure much less readily than their matrix, and the latter, having already a firm consistence, became separated from the crystals, carrying the chlorite with it, and was displaced along the planes which are now cleavage-planes, that is, in a direction at right angles to that of the pressure.