104 OVERLYING AND TRAP ROCKS. 



tient " porphyries" which are merely claystones, 

 clinkstones, and compact felspars, and there are mo- 

 dern ones which are every thing that these are assumed 

 to he. There are basalts and greenstones among the 

 antient porphyries, as there are in the granites ; and 

 even the same mass often contains porphyry, compact 

 felspar, greenstone, Syenite, and even granite of the 

 most genuine character. And thus it must he ; since 

 the composition and origin of the whole is the same, 

 and the differences casual ; as far as casualty depends 

 on unknown causes. 



Hence is there no other philosophical mode of 

 treating this subject than the one which I have adopted. 

 Every other must lead to misrepresentation, assump- 

 tion, or repetitions ; and of the two former there has 

 been more than enough, producing some of the great- 

 est rnisstatements which have hitherto encumbered 

 geology. Let the distinction for granite remain, for 

 the present, what I have assumed, and there will thus 

 be one other, to include all the rocks here associated, 

 under this essential character, that with numerous 

 mineralogical forms, every member somewhere or other 

 " overlies " strata, of different ages, while in other 

 respects they possess the characters of all the Un stra- 

 tified rocks. 



Of the general theory of their origin it is now 

 superfluous to speak : but if there is a predominant 

 granitic aspect or character in certain porphyries of 

 apparently antient dates, and if also they contain 

 more accidental minerals than others which appear to 

 be of later origin, it can probably be explained by the 

 theory already applied to granite and the simpler 

 traps, namely, a slower cooling, permitting the exer- 

 tion of more chemical affinities among the constituent 

 earths. We may not soon precisely know why 



