122 OVERLYING AND TRAP ROCKS. 



while those of a different aspect, connected with the 

 masses that lie on the secondary rocks, pass into the 

 primary strata, the fact of at least a relative antiquity 

 is ascertained ; and thus also there must be masses 

 equally distant in time, since all veins thus originate. 

 In the later ones indeed, it is often easy thus to trace 

 the veins ; while, as in granite, the parent masses 

 may in other cases he invisible, or may have disap- 

 peared in the progress of time ; though, even in the 

 apparently antient, this connexion is often to be dis- 

 covered. These then might be placed in the primary 

 class, if any one is desirous of such a distinction : and 

 they would thus rank, in time, with the granites, 

 with which their essential connexion must be suffici- 

 ently apparent.- 



The same causes render it difficult to determine the 

 number of periods of production intheserocks; though 

 there should have been many, not only in the later 

 traps, but in the " porphyries" of a higher antiquity 

 than the secondary strata. I have shown that, in these 

 cases, no stress can be laid on the circumstance of con- 

 tact with particular strata ; so that, as in granite, we 

 are limited to the test derived from intersecting veins. 

 And when these are found to cross, so that the first is 

 also shiffed by the second, there is evidence of a de- 

 cided posteriority in the latter, while we can equally 

 infer that there have been at least two formations of 

 overlying rock among the primary, as there have been 

 among the secondary strata, and both of them before 

 the foundations of the latter were laid. If there have 

 been more than two of this early date, I know not that 

 I have produced proofs to satisfy myself; yet they may 

 have occurred to others, since the fact is not impro- 

 bable. It is a much more difficult question at present, 

 to. ask whether the granites are all prior to these early 



