Crystalline Structures of Rocks'. 69 



were entertained by Dn Hutton respecting its cause, and from its 

 misapplication in the support of his views. This relates to the 

 ironstones known by the name of septaria, which consist of sphe- 

 roids, generally uniform on the outside, but divided within into 

 polygonal figures, of which the intervals are filled by calcareous 

 spar. It was supposed by him, 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 compound mass ; it 

 having been conceived impossible that it could have entered from 

 without. But the solution of this diflficulty is exceedingly simple; 

 and the occurrence is an obvious instance of the shrinking of a 

 mass of moist earth. In some of the septaria, the external surface 

 is not solid, but the prisms reach it ; and, in these cases, the ease 

 with which carbonate of lime might have entered into the intervals 

 is evident. Where the surface, on the contrary, 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 dissipation of the water 

 through a substance unquestionably capable of permitting its tran- 

 sudation. The subsequent 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 cavities of which are sometimes filled in this manner. 



Of the Spheroidal Structure. 



The spheroidal structure is found under diflTerent modifications ; 

 some of which are among the most inexplicable phenomena of this 

 nature which geology presents. The explanation of those which 

 approach in their nature to crystallization, is not so difficult ; and 

 these examples serve, in some measure, to connect two processes, 

 otherwise very different. 



The large spheroidal structure of granite, already mentioned, 

 cannot perhaps with propriety be ranked with this ; and, for the 

 same reason, I shall omit that which occurs in trap in Rum, al- 

 though a very remarkable occurrence. 



In the secondary sandstones of Egg and other places, there are 

 found large spheroids imbedded in the ordinary strata. These are 



