68 Dr. Mac CuUoch on the Concretionary and 



•which conceives that it arose from the contact of fluid trap with 

 water. That would scarcely explain its nature, even were this a 

 fact proved, which it is not. There is no resemblance between 

 the prisms of trap and those formed by the shrinking of clay : the 

 essential difference lies in the absolute contact of the former, and 

 that objection is insurmountable. To call the arrangement of a 

 basaltic prism crystallization, is, on the other hand, entirely to lose 

 sight of the true nature of this mode of arrangement, which consists 

 in the production of definite geometrical figures 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 crystallization, whether 

 from solution or fusion. 



On the other hand, it appears, that sandstones exposed to heat 

 do assume the prismatic form, while it is certain, that the trap 

 rocks must have often retained their heat long after they had lost 

 their fluidity. 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 sometimes 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 arrangement 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 arrangement exists in that substance, called madreporite 

 limestone, from its resemblance to an organic structure. Respect- 

 ing these, there is nothing further known, from whiph an explana- 

 tion of the causes of these arrangements can be derived. 



There is yet one modification of prismatic structure remaining, 

 which requires notice ; on account of the misapprehensions which 



