474 OVERLYING ROCKS. 



found to differ materially ; the breadth varying* 

 from an inch to nine feet, and the length, from a 

 foot to three hundred or more. 



The columns are sometimes continuous for a 

 considerable space. At others, they are obliquely 

 and irregularly divided by fissures or joints ; 

 and, in some remarkable examples, by transverse 

 ones of various character. In all cases, the dis- 

 tances of these vary materially; the divisions 

 being, in some places, so accumulated as to sepa- 

 rate the column into numerous parts, and, in 

 others, occurring irregularly only after long inter- 

 vals. The forms of the proximate surfaces are 

 either irregular, or flat ; or concave and convex ; 

 and, in some rare cases, a process arises from the 

 angle of each inferior portion, so as to cover a 

 corresponding deficiency in the superior. 



It only remains to observe, with regard to the 

 columnar structure, that it occurs in many mem- 

 bers of this family. It is found in basalt, notedly, 

 but not exclusively ; as it exists in syenite and 

 claystone, in Ailsa, Rum, and Arran, in augit 

 rock, in Sky, in porphyry in Arran and else- 

 where, and in greenstone in various situations. 



