96 



THE GEOLOGIST. 



after many ages had elapsed and hundreds of years had passed away, 

 an immense series of strata was elaborated, which extended continu- 

 ously over the site of the Pentlands. The same waters under which 

 was laid down the ash of the Dry Dam, and which held in solution 

 the substances which fill the amygdaloidal cavities of the greenstones 

 and felstones above it, also deposited above these rocks an enormous 

 series of beds, 3000 ft. thick at least, of sandstone, shale, and lime- 

 stone, the edges of which are seen on the beach eastwards from 

 Portobello. and at East Cairn Hill on the Pentlands, at a height of 

 1839 ft. above the level of the sea, i. e. 1000 ft. above Arthur's Seat. 

 These rocks also extend northwards into Fife. This is represented 

 in the accompanying diagram. 



It is perfectly evident then, from what has been already said, that 

 Lower Carboniferous strata must have been elaborated far above the 

 present altitude of Arthur's Seat, and that their area extended for a 

 considerable distance on all sides, overlying the whole ridge of the 

 Pentlands. Tt is also pretty clear, both from the horizontal beds of 

 sandstone on the summit of East Cairn Hill, and from the state of 

 the case generally, that these strata were at first laid down in a hori- 

 zontal position, or at all events very nearly so. 



Now, the next stage through which these rocks passed is easily de- 

 termined. An upheaving movement must have taken place, by which 

 the whole beds of the hill, already alluded to as forming the eastern 

 part, on to the centre of the Musselburgh basin, were raised into the 

 inclined position in which we noiv find them. If evidence of this up- 

 heaval is wanted, we have only to point to St. Leonard's Craig, Salis- 

 bury Craigs, with the two dykes cutting through them, and the Dasses, 

 the only intrusive traps of the. hill, and all found on the western side. 

 The effects of this upheaval are well displayed at Salisbury Craigs, 

 where the beds have an easterly dip, being raised on the west and 

 depressed on the east, and where both the sandstone and the green- 

 stone are altered in colour and texture at the point of contact. In- 

 deed, nothing shows the intrusive character of this trap more clearly 

 than the seam of shale to be seen at the south end of the Craigs, very 

 much hardened and broken, lying completely enveloped in the green- 

 stone. After this eruption, then, the rocks of the hill would present 

 somewhat the appearance given in the accompanying diagram, PL V. 



But this is not the present aspect of the hill ; Carboniferous strata 

 no longer conceal the trap-rocks from view. How, then, have they 

 disappeared ? What has become of them? The answer is, denuda- 

 tion must have taken place. Now I think it is a fact winch can be 

 easily demonstrated, that Arthur's Seat has been subjected to denu- 

 dation at two different periods. 



First Period. — After the upheaving of the strata, already referred 

 to, the beds, of course, were set on their edges (as in preceding 

 figure), and were much fractured. The faults, too, which are so 

 abundantly met with in the coal-measures, were no doubt produced 

 by this upheaval. Consequently, in this state, the strata were 



