rKANSACTIONS OF THE SECTIONS. 89 



•were ejected the basalts and coarse agglomerates of tlie summit and shoiilders of 

 Arthm-'s Seat. There is no trustworthj' evidence for fixing the geological date of 

 this eruption. Evidently, from the gi-eat denudation by which it was preceded, 

 it must belong to a much later period than any of the Carboniferous eruptions. 

 Yet, from the gTeat similarity of the Arthiu-'s Seat agglomerate, both in compo- 

 sition and mode of occurrence, to numerous " necks " which rise through all parts 

 of the Carboniferous system between Nithsdale and Fife, and which I have shown 

 to mark the position of volcanic orihces during Permian times, I am inclined to 

 regard these later igneous rocks of Edinburgh as dating from the Permian period. 

 Ai'tluu-'s Seat, however, seems to have been the onlj' volcano in action during that 

 period in this neighbourhood. 



There still remains, for notice one further and final feature of the volcanic his- 

 tory of this part of Scotland. Rising inditferently through any part of the other 

 rocks, whether aqueous or^igneous, and marked by a singular uniformity of direction, 

 there is a series of basalt dykes which deserves attention. They have a general 

 easterly and westerly trend, and even where, as in Linlithgowshire, they traverse 

 tracts of basalt-rocks, they preserve their independence, and continue as readily 

 separable as when they are found intersecting sandstones and shales. These dykes 

 belong to that extensive series which, running across a great part of Scotland, the 

 north of England, and the north-east of Ireland, passes into, and is intimatety con- 

 nected with, the wide basaltic plateaux of Antrim and the Inner Hebrides. They 

 date, in fact, from Miocene times, and, from their numbers, their extent, and the 

 distance to which they can be traced from the volcanic centre of the north-west, 

 they remain as a striking memorial of the vigour of volcanic action during the last 

 period of its manifestation in this country. 



Glacial Phenomena. 



To an eye accustomed to note the characteristic impress of ice-action upon a 

 land-surface, the^ueighbourhood of Edinburgh presents many features of interest. 

 It was upon Corstorphine Hill, on the western outskirts of the city, that Sir James 

 Hall first called attention to striated rock-surfaces which, though erroneously at- 

 tributed to the abrasion produced by torrents of water, were even then recognized 

 as trustworthy evidence of the last great geological changes that had passed over 

 the surface of the countrj'. Even before we come to look at the sm'face in detail, 

 and note the striation of its rocks, we cannot fail to recognize the distinctively ice- 

 worn aspect of the hills round Edinburgh. Each of them is, in fact, a great roche 

 inoutuunee, left in the path of the vast ice-sheet which passed across the land. 

 That this ice was of sufiicient depth and mass to override even the highest hills, 

 is proved not merely by the general ice-worn surface of the landscape, but by the 

 occurrence of characteristic striaj -on the summits of the Pentland Hills, 1600 feet 

 above the sea ; that it came from the Highlands, is indicated by the pebbles 

 of granite, gneiss, schist, and quartz rock occurring in the older boulder-clays 

 which it produced ; and that, defiected by the mass of the southern uplands, the 

 ice in the Aalley of the Lothians was forced to move seawards, in a direction a little 

 north of east, is shown by the trend of the strins gTaven on the rocks, as at Cors- 

 torphine, Grantou, Arthur's Seat, and Pentland HUls. 



Connc.viun of the present form of the Surface with Geological Structure. 



In concluding these outlines, let me direct the attention of the Section to the 

 bearing which the geological structure of the district wherein we are now assem- 

 bled has upon the broad and much canvassed question of the origin of land- 

 surfaces. In the first place, we cannot fail to be struck with the evidence of 

 enormous denudation which the rocks of the district have undergone. Every for- 

 mation, from the oldest to the latest, lias suffered, and the process of waste has 

 been going on apparently from the earliest times. We .see that the Lower Silu- 

 rian rocks were upheaved and denuded before the time of the Lower Old Eed 

 Sandstone ; that the latter formation had undergone enormous erosion before the 

 beginning of the Carboniferous period ; that of the Carboniferous rocks, a thick- 

 ness more than 3000 feet had been worn away from the site of Arthur's Seat be- 

 fore the last eruptions of that hill, which are possibly as old as the Pennian period 



