Crystalline Structures of Rocks. 67 



sandstones, but, of course, on a small scale. There is, in this case, 

 no shrinking, as in dried clay, to account for the appearance ; the 

 sides remaining in perfect contact, just as in the columnar traps. 

 The same circumstance has been observed by Mr. Chantrey, in 

 those sandstones which are heated for the purpose of making roads 

 in Derbyshire. Here therefore it is directly proved, that heat is 

 capable of inducing the prismatic structure in a solid sandstone ; 

 and, that this is not the development of an original concretionary 

 structure, is proved by the fact, that in the hearthstones which 

 have undergone this change, the arrangement of the prisms is 

 always vertical to the plane of the stone ; a remarkable analogy 

 to their mode of arrangement in the trap rocks. 



Ignorant as we are of the nature of the concretionary structure, 

 it is still certain that it bears a kind of analogy to crystallization ; 

 and the well known experiments of Mr. Watt, prove that this 

 arrangement, if it be not rather a concretionary one, may take 

 place in rocks without fluidity. It is also known, that a curved 

 structure is sometimes developed 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 impossi- 

 ble to pursue this argument further, for want of a greater store of 

 facts. It must be left to make that impression on unprejudiced 

 readers, which is all that an imperfect train of reasoning is entitled 

 to expect. 



Yet, in terminating 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 ; invisiWe from want 

 of contrast of colour or texture, and only developed on wasting. 



It remains to enquire whether this fact may not be analogically 

 extended to account for the columnar forms 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 



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