Prismatic Structure in Sandstone, Sfc, 253 



of the different substances to enter into new combinations, it 

 generates the crystalline structure ; and here, also, we may 

 observe that there is an analogy between the effects, the same 

 cause producing the crystalline structure where the rock is 

 fluid, and the concretionary where it does not lose its solidity. 



If, therefore, the limited experiments which have been 

 hitherto made in our laboratories have thrown light on the 

 crystalline structure of certain rocks, by proving that this may 

 have taken place from a state of igneous fluidity, the accidental 

 experiment here described may, perhaps, tend to throw a 

 similar light on the concretionary structure of rocks. The 

 experiment, indeed, is as yet solitary; but even a solitary 

 experiment is the result of a law, and that law must act in 

 every other case, where the circumstances are the same. We 

 may therefore infer, without much hazard of error, that the 

 long-continued application of a certain degree of heat to a 

 sandstone of certain composition, will, in all cases, under 

 identical circumstances, generate in it the prismatic concre- 

 tionary structure. It will hereafter be shown, in the second 

 part of this paper, how this principle may be applied to the 

 solution of some difficult cases in geology. 



How far we may venture to extend this principle from sand- 

 stone to other rocks, is a question to which no decided answer 

 can be given. It may easily be conjectured that I here allude 

 to the trap rocks, which present examples of the concretionary 

 structure the most remarkable and frequent among those with 

 which we are acquainted. 



To illustrate this subject as it merits, it would be necessary 

 to put it to the test of direct experiments modelled on that 

 which is here recorded. What time would be necessary to 

 produce a result, if a result is to be produced, is uncertain. I 

 have attempted in vain, by inquiries among iron-founders, to 

 learn what length of time the exposure to heat was required to 

 produce the prismatic configuration of the hearth sandstone. 

 But the destruction of a furnace is a rare occurrence, and the 

 time which it remains at work is such, that there seems no 

 possibility of discovering the minimum of time required for 

 this effect. Still, the experiment might be tried in the hands 

 of those who are conveniently situated for that purpose, by 



S2 



