Granite of Aberdeenshire. 49 



yet occurred of substances resembling clinkstone ; and if the 

 amygdaloidal structure never occurs in that rock, that circum- 

 stance is easily explained by the peculiar conditions necessary 

 for the production of that cavernous structure from which the 

 amygdaloidal seems to arise. That a large proportion, at least, 

 of the amygdaloidal nodules, are the result of a subsequent 

 infiltration, is proved by circumstances which I have stated in 

 other places, but into the details of which I cannot here enter. 



In comparing, finally, the mineralogical differences of these 

 two classes of rock, it must be observed, that they consist more 

 in the relative proportions of the several varieties in each, than 

 in the different characters of those members, separately con- 

 sidered. In granite, the well-known compounds to which this 

 name is generally applied, abound almost to the exclusion of 

 those which have here been described, and that resemble the 

 rocks of the trap family. In this latter division, on the con- 

 trary, greenstone, basalt, and claystone, are among the pre- 

 vailing substances ; while the compounds that resemble granite 

 are very rare. 



But, that too much stress may not be laid on the differences 

 which have here been pointed out, it is proper to remark, that 

 the several members of the trap family, differ as much among 

 each other, as the whole, collectively taken, differs from the 

 rocks that rank under granite. Even in comparing the individual 

 members, tlie contrast between the softest claystone and the 

 syenite of Sky, or between that simple rock and the numerous 

 porphyries which are found in this family, is as great as that 

 which exists between the same substance and granite. 



It is not one of the objects of this paper, to protract this ex- 

 amination of the analogies and differences between granite and 

 the trap rocks, further. To enter more deeply on the discus- 

 sion, would require a space exceeding^* the limits assigned for 

 it ; since it would be necessary, among other matters, to point 

 out all the circumstances in the origin of both, and all tlie 

 probable causes which, in either, might have produced those 

 appearances which are not at all to be found, or exist more 



Vol. X. E 



