EXCURSION II. 81 



splitting into prisms. The bed is very high 011 the cliff, 

 coming close up to the prismatic greenstone, but apparently 

 separated everywhere by altered sandstone. Its upper sur- 

 face is on a level with some of the depressions in the edge of 

 the trap ridge; and it is apparently through the most eastern 

 of these that this bed passes across to the back of the ridge, 

 and is again seen behind Dun-fion, where a bed of sandstone 

 is also seen. It no doubt passes downwards towards the bay, 

 but it is not seen again : perhaps the trap of the cliffs is a 

 mere facing, and the pitchstone may traverse the sandstone 

 only. It is the upper surface of the bed that is here 

 exposed. 



The lower pitchstone vein, c, is fourteen feet thick, and of 

 the same colour and structure as the other ; but its low level 

 can hardly admit its passing across the ridge. The vein of 

 porphyry occupies a ledge about fifty yards lower down : 

 this ledge being on the level of the front of Dun-Dhu, from 

 which the porphyry extends. The sandstone strata are not 

 well seen on the ledges a a a, between the beds, in the line 

 of section, but come out distinctly on their continuations 

 east and west. 



Neither of these pitchstone beds has any connection with 

 those upon the shore, which are far below the level of the 

 base of our section. But the upper bed probably passes 

 westwards across the hollows south of Dun-Dhu, since a bed 

 of pitchstone, about twelve feet wide, again appears on the 

 west bank of the first burn north-west of this hill, and in a 

 position to which a slight undulation in the upper bed would 

 bring it. A few hundred yards farther west, in a field 

 within a high fence, it breaks out again with a breadth, of 

 more than twenty-five feet, and, undulating again, passes Tip 

 westwards by the crest of the moor, where it appears of 

 considerable breadth, there being only indications of its 

 occurrence between these two points. But from the crest 

 of the moor it can be traced continuously into Birk Glen. 

 (See Excursion IV.) The rock is most usually glassy, of a 



