138 ON THE CHARACTERS AND DISPOSITION 



boundaries of those strata which have certainly been 

 deposited under the ocean. Hitherto, they terminate 

 with the chalk stratum. At the same time, the 

 masses of that rock lie above this substance, and thus 

 its posteriority to all the strata beneath the tertiary 

 deposits,, taking chalk as the uppermost of these, is 

 established. But as trap occupies the surface, and as 

 the upper strata in the general series are not found at 

 every point there, it may of course happen, that instead 

 of chalk, this rock may repose on any other member 

 of the series, as it may even on granite. It must 

 already be apparent, that the mere existence of trap 

 veins in granite or gneiss, cannot prove their anti- 

 quity; unless it can be shown at the same time, that 

 they stop suddenly at the point of superposition of 

 some other rock. Neither can the separate super- 

 position of a mass of the same substance on those, or 

 on any other rocks, prove their affinity to these in 

 point of age. That can be inferred only in cases of 

 continuous stratification among the stratified sub- 

 stances; and, if the contrary opinion has been held, 

 it has arisen from a false view of the nature of the 

 trap rocks. Were a continuous mass of trap to be 

 deposited now over the whole of Britain, it would be 

 in contact with every rock in the system. Should 

 certain portions of it disappear through the lapse of 

 ages, there would remain others, independent, and, 

 under occasional circumstances, in contact with granite^ 

 with argillaceous schist, or with chalk, as it might 

 happen. Posterity would judge falsely in considering 

 these as distinct deposits, connected, in point of age, 

 with the several rocks on which they immediately 

 repose, and through which each of them would also 

 exclusively send its veins. But this is a picture of 

 the past, as all geology proves; and we arc therefore 



