OVERLYING AND TRAP ROCKS. 123 



examples, and what arc the relations of date among 

 the several granites and the different porphyries. I 

 have never seen a granite vein traversing a porphyry, 

 and I know not that any one else has; while if this he a 

 fact, the oldest porphyry is more recent than the latest 

 granite. Yet I fear that this will ever he one of the 

 insuperable difficulties in this great family of the un- 

 stratified rocks ; because, under what I have shown 

 respecting the frequent community of character be- 

 tween granite and trap, under the changes which a 

 granite mass undergoes in its veins, under the fact that 

 actual porphyry does occur in granite, as a casual 

 change in the crystallization, independently of that 

 gradation between the two in the cases of porphyritic 

 granite and granitic porphyries, and lastly from what 

 we know of the chemical causes of these variations, 

 we never can be sure that a mass or a vein of por- 

 phyry may not be the mere local variation of a granite. 

 And thence perceiving that geological difficulty which 

 geologists have never yet contemplated, because they 

 have ever observed and reasoned as mere mineralogists, 

 upon specimens and not on geological principles and 

 causes, the reasons for treating of all the unstratified 

 rocks as one family become still more apparent, if we 

 intend to investigate geological science, and not to col- 

 lect and examine mineral masses, imagining at the 

 same time that we are learning and teaching Geology. 

 If, equally, among the traps in the secondary strata, 

 no proof of two or more deposits is derived from their 

 contact with beds of different dates, I must now re- 

 mark that the superposition of one mass above another, 

 even if of different kinds, affords none. These may have 

 been made in succession during one interval ; while 

 differences of composition and character equally prove 

 nothing, since they occur, in a mixed and graduating 



