OVERLYING AND TRAP ROCKS, 137 



The columns vary in the number of angles, from 

 three, even to eleven or twelve; the medium polygons, 

 from five to seven being prevalent, while, in the higher 

 ones, the angular forms almost vanish. In all 

 cases, the sides are in contact, or so little separated as 

 barely to admit of the infiltration of carbonat of lime, 

 if present; a striking point of difference, as formerly 

 observed, between them and the prismatic bodies 

 which result from the cracking of dried clay. No 

 case of a single polygon thus aggregated has yet been 

 observed; the various forms occurring together, and 

 the adaptations being produced by the combination 

 of many different prisms. The columns are sometimes 

 continuous, at others jointed, obliquely or at right 

 angles: being also ocasionally cracked, without the ap- 

 pearance of regular joints. These, when they exist, vary 

 much in frequency; being sometimes repeated within 

 the space of a few inches, at others, only after many 

 yards. At times they are mere divisions, the surfaces 

 above and below remaining flat; while, at others, they 

 are irregularly undulated, and, more rarely, adapted 

 by a mutual convexity and concavity ; the latter gene- 

 rally belonging to the lower surface. In a few rare 

 instances, as in the Giants' Causeway, each angle of 

 the inferior joint is prolonged upwards into a denticle 

 overlapping that of the superior one. Their sizes 

 vary exceedingly, both with respect to length and dia- 

 meter: the longest which I know, in Sky, reaching 

 to about 400 feet, and the shortest, in Morven, not ex- 

 ceeding an inch; while in diameter, those of Ailsa 

 measure nine feet, and those of Morven, an inch, or 

 less. Intermediate sizes are, of course, more prevalent. 



With respect to position, they are found placed in 

 every manner, from the horizontal to the vertical angle, 

 though attracting most attention in these latter cases, 

 from their resemblance to the efforts of architecture 



