176 ON THE CONCRETIONARY AND 



by the repeated addition of particles of a definite 

 form, whether these be simple atoms or compounded 

 chemical molecules. In the prisms of trap, the laws 

 of geometry and chemistry are equally violated ; and 

 the objection applies equally to both modes of cry- 

 stallization, whether from solution or fusion. 



On the other hand it appears, that sandstones ex- 

 posed to heat do assume the prismatic form, while it 

 is certain that the trap rocks must have often re- 

 tained their heat long after they had lost their flui- 

 dity. It is unnecessary to draw out the argument 

 further. The prismatic form might have occurred 

 even after the rock was consolidated: if any additional 

 facility is gained by the supposition, this change may 

 be conceived to have gradually taken place while a 

 state of tenacity still permitted a certain degree of 

 motion among the parts. 



A small and irregular prismatic disposition is some- 

 times found in the pitchstones, as well as among the 

 traps ; and it can scarcely be considered as more 

 than a modification of the laminar form into which 

 it passes. In certain argillaceous ironstones and 

 jaspers, there has also been observed a prismatic ar- 

 rangement on a small scale ; which is further often 

 singularly marked by protuberant joints, or by small 

 stripes or channels parallel to the prisms. A similar 

 disposition exists in that substance, called madreporite 

 limestone, from its resemblance to an organic struc- 

 ture. Respecting these, there is nothing further 

 known, from which an explanation of the causes of 

 these arrangements can be derived. 



There is yet one modification of the prismatic 

 structure remaining, which requires notice ; on ac- 

 count of the misapprehensions which have been 

 entertained respecting its cause, and from its rnisap- 



