466 OVERLYING ROCKS. 



and, in that light, they may be considered uni- 

 versal. Yet thev cannot be considered universal 



V 



in the same wide sense as granite, the primary 

 strata, and the principal members of the secon- 

 dary stratified rocks. They appear, on the con- 

 trary, to be in a great measure limited to parti- 

 cular spots, more or less extensive, and to be, if 

 separately considered, partial and independent 

 productions. 



Where they do occur, they sometimes rise 

 into high mountains, of which, in this country, 

 Sky and Mull afford examples ; at others, they 

 form lower hills, or even summits absolutely in- 

 dependent. These masses are generally irregu- 

 lar, but sometimes bear indistinct marks of 

 stratification ; the vertical edges of the successive 

 beds producing that scalar appearance in the out- 

 line of the land whence the name Trap has been 

 derived. They also frequently occur in veins ; 

 and these are sometimes connected with the 

 larger masses, while, at others, they appear to be 

 absolutely independent of any visible larger por- 

 tion or tract. 



Under the uncertainties already stated, res- 



