of the Structure qj ( Rocks. 377 



ment be taken in its literal meaning, I am not prepared to 

 concede that all mineral masses have experienced structural 

 changes since the period of their consolidation. The point 

 at issue between us, in this instance, may be thus stated : the 

 Professor supposes that rocks have acquired some kinds of 

 structure by the application of mechanical force — sometimes 

 external — since they have been in a solid state; whereas, on 

 the other view of the subject, all kinds of structure are ac- 

 complished when rocks become perfectly solidified. 



A detailed consideration of the subject will, however, more 

 clearly show the merits of either opinion ; and, in the first 

 place, the structure of the granitic, trappean, and other igneous 

 rocks will engage our attention. 



" No one supposes," observes the Professor at page 46 L, ubi 

 sup. " that columnar basalt was originally built up of solid pa- 

 rallel jointed pillars, or that the structure of a granitoid rock 

 was effected by a mere fortuitous concourse of the crystalline 

 parts. We believe that these phenomena are the necessary 

 consequences of a certain anterior condition of the materials. 

 . . . . The mass which has changed its temperature, and become 

 solid, has also changed its dimensions. Contraction must 

 produce tension on the whole mass; and this tension, acting 

 mechanically, will in many instances produce joints and fis- 

 sures, and sometimes contortions." On this it may be re- 

 marked, that the peculiar geometrical forms of these rocks 

 have certainly not been produced by a mechanical nor by a 

 fortuitous arrangement'; but have probably, at the time of con- 

 solidation, resulted from a process very similar to, if not iden- 

 tical with that of crystallization : and further, it may also be 

 maintained, that the joints are the bounding planes of such 

 crystals, having, therefore, not a posterior, but a synchronous 

 origin with that of the solid concretions. Now, these joints 

 may be more or less distinctly defined, as in the crystals of 

 various simple minerals; and may have been rendered more 

 strongly marked by open spaces and other alterations, subse- 

 quently effected by the operation of mechanical and chemical 

 causes ; but the occurrence of such fissures does not neces- 

 sarily imply that the joints of rocks have been occasioned by a 

 tension arising from contraction of the mass during cooling. 

 Indeed, we know little experimentally concerning the con- 

 traction of rocks in passing from the fluid to the solid state ; 

 and it is even possible that some of them, like other bodies, 

 may actually expand during such a change of condition. 

 Even admitting that contraction does take place, we have yet 

 to learn that it would operate on innumerable small portions, 



Third Series. Vol. 7. No. 41, Nov. 1835. 3 C 



