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 corn- 

 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 .thus 

 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 

 jn the ammonites containing calcareous spar, is abun- 

 dantly obvious. 



VOL. \, * 



