CRYSTALLINE STRUCTURES OF ROCKS. 175 



in rocks by heat. The present discussion may 

 perhaps render it doubtful whether this is not rather 

 the generation of a concretionary structure. It is 

 impossible to pursue this argument further, for want 

 of a greater store of facts. It must be left to make 

 that impression which is all that an imperfected 

 train of reasoning is entitled to expect. Yet, in ter- 

 minating these observations, it is right to remark, 

 that the decided union of the concentric arrangement 

 with the prismatic form in the sandstone of Dunbar, 

 renders it probable that this arrangement exists also 

 in the prisms of trap ; invisible, from want of con- 

 trasts of colour or texture, and developed only on 

 wasting. 



It remains to inquire whether this fact may not be 

 analogically extended to account for the columnar 

 form of the trap rocks. Different causes have been 

 assigned for this by geologists. It has been supposed 

 to result from the division of a mass of a soft and 

 moist material, by drying and consequent shrinking ; 

 and it has been attributed to crystallization, from a 

 state of igneous fluidity, or from solution in water. 

 It is useless to examine that theory 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, as I have already re- 

 marked, 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 con- 

 sists in the production of definite geometrical figures 



