252 Dr. Mac Culloch on a 



yet discovered, without end. In the concretionary structure, 

 no such divisibility occurs ; but the pseudo-crystalline form, 

 which it assumes, is only divisible into shapeless parts, which 

 bear no relation to each other, or to the general figure of 

 which they are the constituents. Yet it is probable, that even 

 the crystal must be constituted in the same manner ; and that 

 the ultimate particles, or atoms of matter of which it is com- 

 posed, have no forms coincident with that of the molecule 

 which they generate. 



Whatever the differences between true crystallization and 

 the concretionary structure in rocks may be, there is, at any 

 rate, an analogy sufficient to show that both arrangements have 

 been produced by certain attractions among small particles, 

 tending to arrange them in particular forms. In the crystalline 

 structure, that attraction has been exerted, either among simple 

 atoms, or among definite combinations of these, free to act 

 and move in the directions to which they are impelled by 

 certain unknown laws, in consequence of the fluidity of the 

 mass of which they formed the parts. In the concretionary 

 arrangement, or the contrary, from every thing as yet known, 

 the motions of the parts have been limited ; either by their 

 figure, bulk, or imperfect capability of motion ; or in conse- 

 quence of previous determined combinations, perhaps of a 

 more complicated nature, and of the want of a fluidity, enabling 

 them to assume the disposition to which they might possibly 

 have tended under more favourable circumstances. 



It is impossible, at present, to illustrate this obscure subject 

 further ; but it is evident, that on examining the various expe- 

 riments that bear on this question, the action of fire is capable 

 of producing both the crystalline and the concretionary arrange- 

 ment. 



In simple crystallizable bodies, heat changes the crystalline 

 arrangement, even though the substance remains solid : in the 

 case of this sandstone, it has generated the concretionary 

 structure. In glass, it seems to produce a crystalline arrange- 

 ment also ; but, in this case, it is probable the change does 

 not resemble that which takes place in metals, but that the 

 nature of the combination of the integrant substances is altered. 

 In fluid rocks, the parts being free to move, and the particles 



