OVERLYING ROCKS. 471 



ter. This modification of structure is not pecu- 

 liar to those varieties which seem among the more 

 antient, such as hypersthene rock, in which it is 

 very conspicuous ; as it occurs in the Corstor- 

 phine hills near Edinburgh, in a formation which 

 lies above the coal strata. 



The next, and by far the most remarkable 

 variety of structure, is the prismatic. The first 

 tendency to this, is seen in a vertical mode of 

 fracture assumed by the exposed edges of these 

 rocks, which at length becomes more decidedly 

 prismatic, and ultimately columnar. When 

 the mass possesses a parallel disposition, and is 

 at the same time horizontal, while the prisms are 

 at right angles to it, the well-known effect of 

 architectural regularity, so conspicuous in Staffa, 

 is produced. 



This columnar structure, however regular it 

 may be in a particular place, sometimes vanishes 

 in the same mass, either laterally, according to 

 the repetition of the prisms, or vertically ; any 

 individual column, or group, being found to be- 

 come imperceptibly amorphous. In other cases, 

 it is found that columns are irregularly dispersed 



