472 OVEKLYING ROCKS. 



in various directions through an amorphous mass, 

 of which the upper part of Staffa also presents a 

 good example. In a similar manner, a solid rock 

 is sometimes merely split at its surface into 

 short prismatic divisions ; but this modification 

 is comparatively very rare. 



If the mass is inclined, or not horizontal, the 

 prisms which are at right angles to it, can no 

 longer he vertical, and thus, in vertical veins, 

 they are sometimes horizontal. But columns are 

 not necessarily at right angles to the parallel 

 masses ; since, in various places, and conspicu- 

 ously in Staffa, they lie in several irregular po- 

 sitions in the same bed, even where in contact 

 with each other. 



Prisms thus irregularly placed, sometimes in- 

 terfere \\ith earh other so as to lose their regu- 

 larity. This happens in such a manner in Canna, 

 that they bear a general resemblance to that fa- 

 miliar object, a peat-stack ; while, in other places, 

 they present other modifications of irregularity 

 on which it is unnecessary to dilate. In veins 

 also, they are not necessarily transverse to the 

 walls ; since, in Rum and Cantyre, they occur in 



