n 



94 



ILLUSTRATIONS OF THE 



Hill covered by the flrata in their natural place 

 and has been perfedly defended by them from 

 every fort of wearing and decay. The cunei- 

 form ftiape, therefore, which this rock takes at 

 its extremities, and the great difference of its 

 thicknefs at them and in the middle, is a part 

 of its original conftitution, and can be attributed 

 to nothing cafual, or fubfequent to its confolida- 

 tion. 



The fame may be faid of many other bafaltic 

 rocks, where an inequality of thicknefs, mod 

 unlike to what belongs to aqueous depofites, is 

 I^nown to exift in beds of whinftone that are 

 Hill deep under the furface. Thus the toadftone 

 of Derbyfhire, even where it has a thick cover- 

 ing of ftrata over it, has been found, by the fink- 

 ing of perpendicular fliafts, to vary from the 



thicknefs of eighteen yards to more than fixty. 



within the horizontal diflance of lefs than a fur- 



long. 



Nothing of this kind is ever found to 



V 



take place in thofe beds of rock v/hich are cer- 

 tainly known to originate from aqueous depoli- 

 tion, and no character can more ftrongly mark an 

 elTential difference of formation. 



258. We have had frequent occafion to con- 

 fider the charaders of thofe mafles of whinflone 

 which are fo often found interpofed between 

 ilratified rocks. Thefe have been found in ge- 

 neral very adverfe to the Neptunian fyftem ; and 



two 



