102 



CHAP. XXV. 



Overlying and Trap Rocks. 



THE preceding tabular Classification is a necessary- 

 portion of a System of Geology, and a ground of 

 reference for the rocks here described. But, having 

 taken different views of what the science demands, I 

 shall not here follow it : while if geologists have per- 

 sisted in treating of the later trap rocks after the 

 secondary strata, it has not been from a correct theory, 

 but from following that German arrangement which I 

 have discarded. It is true that there are trap rocks 

 later than even the chalk ; but they are also of all 

 ages, and cannot therefore be separated by geological 

 dates without leading to numerous repetitions. Mi- 

 neralogically, there is no such distinction between the 

 apparently oldest porphyries and the demonstrably 

 newest ones, as to allow us to connect these with their 

 geological ages ; while there are some as antient as 

 granite, or older at least than any secondary stratum. 

 If they are therefore to be united in one family of un- 

 stratified rocks, as they must be, they follow it as their 

 most fitting place. If, also, after an examination of 

 granite and trap, as new as I believe it to be just, I 

 have succeeded in showing that they cannot be effec- 

 tually distinguished as mineral compounds, and that 

 their geological distinctions are also evanescent, or un- 

 assignable except under a conventional arrangement, 

 the same, even more frequently, is true of the oldest 

 porphyries and the newest traps, here united in one 

 division ; while for want of that criterion of ages al- 

 ready shown not to exist in these rocks, except in 

 casual instances, we are often as unable to assign the 

 probable date of an antient porphyry as of a recent 



