148 ON THE CHARACTERS AND DISPOSITION 



superficial strata, is fully proved by the phenomena 

 attending the volcanic Coral islands. 



It is in the deeper regions of the globe, therefore, 

 in those where we have found the origin of granite, 

 that we must seek that of trap. These substances 

 are essentially of the same nature, but they have been 

 produced at distant periods of time. In accounting 

 for the present superficial position of trap, we are 

 provided with two resources; that of its flowing out 

 in the manner of lava, so as to cover the strata, and 

 the final removal of these so as to leave bare that 

 which was once concealed beneath them. This ques- 

 tion is worth pursuing a little further. 



If chalk be the uppermost marine stratum in any 

 spot, and if a trap has been erupted under the sea, as 

 many appearances indicate, it must have flowed over 

 the surface of that rock. It may have flowed above 

 many others ; as it is by no means certain, nor even 

 probable, that chalk has been formed every where. 

 But respecting this we cannot hope to acquire any 

 satisfactory evidence; as, in the present state of the 

 earth's surface, it is impossible to determine which 

 strata have been removed, and which were never de- 

 posited. 



On this supposition we may also account for the 

 partial existence of the trap rocks. It is not necessary 

 to suppose that they have been universal, or even 

 much more general than their present remaining indi- 

 cations now show. Although the materials of trap 

 may have existed beneath, as generally as those of 

 granite have been supposed to do, it does not follow 

 that they should every where have reached the surface. 

 The analogy of volcanoes here comes again to out- 

 aid; indicating, by facts which cannot be disputed, 

 the possibility, or rather the probability, of this sup- 



