We looked at 2 exposures of basalt alongside the Queen’s Road (locality 15), the first showing smoothing by passage of ice and the second scratching and polishing along a fault plane (slickensides).

 The main vents for the eruptions of Carboniferous times on Arthur's Seat were at the Lion’s Head and the Lion's Haunch:

....here, we noticed the two principal rock types to be found in the heart of the volcano – dark, fine-grained basalt and crumbly agglomerate, the latter often containing volcanic bombs and mixed sedimentary material caught up in the vent: 

agglomerate at right

To the north-west was the sill intrusion of Salisbury Crags:

... and to the north-east,  a succession of lava flows, all dipping at about 25 degrees to the east.  Some of the lava flows are likely to have derived from the Edinburgh Castle vent rather than from Arthur’s Seat.

[Localities refer to those in the leaflet "Discovering Edinburgh's Volcano", published by Edinburgh Geological Society]