. 94 



mediate succession, when, in others, the two are 

 separated by different intervening strata. It is 

 however doubtful whether observations have as yet 

 been sufficiently extended to j ustify the adoption 

 of any invariable laws on this subject. 



With respect to the trap rocks, it is now un- 

 necessary to observe, that they are found in con- 

 tact with every substance in the catalogue. 



