OVERLYING ROCKS. 473 



a parallel position ; being vertical in the former 

 case, and horizontal in the latter, in veins that 

 are themselves vertical. 



In almost every instance, the adjoining prisms 

 are in contact, without any intervening substance 

 or interval. In one case alone, which occurs in 

 Rum, the prisms which are found on the surface 

 of a large spheroidal mass, are separated by con- 

 siderable intervals, which contract in the progress 

 downwards till the parts touch. 



In many remarkable examples, the prisms, or 

 columns, even for a considerable length, are 

 straight, or nearly so. In others they are slightly 

 incurvated, and, in a few rare cases, as in Staffa, 

 they are much bent, and often, at the same time, 

 in various directions. 



The forms of the columns differ; and the 

 number of the sides, where they are most regular, 

 vary from three to twelve ; but they are more 

 frequently of four, five, six, and seven ; these 

 being intermixed in such a manner, as to pro- 

 duce the absolute contact of the whole. Those 

 sides are not always necessarily straight ; but are 

 occasionally curved. In size, they have been 



