136 OVEllLYING AND TRAP ROCKS, 



and claystones. It is sometimes such as to present an 

 imperfect prismatic appearance ; whence it gradually 

 passes into the complete columnar structure, the most 

 remarkable of all the modes of configuration presented 

 by any rocks, which, though already generally de- 

 scribed, requires some further detail here. The columns 

 sometimes adhere so as to be inseparable by mechani- 

 cal force ; though ultimately separated by the action 

 of the weather. In these examples, they sometimes 

 pass, either end-wise, or laterally, into the amorphous 

 or the prismatic rock ; while that portion only which 

 constitutes the visible surface presents the columnar 

 appearance. .At others, the columnar portions form 

 distinct beds, separated by intersection from the amor- 

 phous part, both above and below ; while a columnar 

 bed is even sometimes cut through at right angles by 

 a similar intersection ; the adaptations of the columns 

 still continuing so perfect, that the direction of a single 

 column remains unaffected. It is equally remarkable, 

 that where portions of the secondary strata are entan- 

 gled among columnar beds, as in Sky, that continuity 

 of character is not affected, nor the regularity or forms 

 of the columns changed, even at the points of contact. 

 But these columns are sometimes also perfect, and easily 

 separated by sufficient mechanical force ; their forms, 

 in this case, displaying different degrees of regularity. 

 In a few instances, being geometrically straight and 

 true in all their parts, they present those beautiful ob- 

 jects, too often and familiarly described to require no- 

 tice here. Sometimes, the columnar parts, instead of 

 being parallel and in one plane, are so situated that the 

 termination of one is approximated to the middle of 

 another; so that the extremities are compressed and 

 extenuated, as if they had been forced together when 

 in a soft state; the whole mass bearing a lively resem- 

 blance to a peat stack. 



