CRYSTALLINE STRUCTURES OF ROCKS. 177 plication toward the support of the views of certain speculating geologists. This relates to the iron stones known by the name of septaria, which consist of spheroids, generally uniform on the outside, but divided within into polygonal figures, of which the intervals are filled by calcareous spar. It was sup posed, that these stones had experienced the influence of fire, and that, in the act of consolidation, the cal careous matter had been separated from the com pound mass ; it having been conceived impossible that it could have entered from without. But the solution of this difficulty is exceedingly simple; and the occurrence is an obvious instance of the shrink ing of a mass of moist earth. In some of the sep taria, the external surface is not solid, but the prisms reach it; and, in these cases, the ease with which carbonat of lime might have entered into the in tervals is evident. Where the surface, on the con trary, is unbroken, it is no less easy to understand how, during the drying of such a nodule of clay, that part would first consolidate; while the interior would necessarily shrink and split, from the dissipa tion of the water through a substance unquestionably capable of permitting its transudation. The sub sequent infiltration of lime into the cavities ithus formed, is not only easy to apprehend, but is a fact of daily occurrence in rocks of a far more compact nature, namely in the traps; the amygdaloidal ca vities of which are filled in the same manner. The resemblance of this process to that which takes place in the ammonites containing calcareous spar, is abun dantly obvious. .3* von. i