The geological structure of Arthur Seat has long been well known. It served as a theme for discussion in the Neptunist and Plutonist controversy, and was often referred to in the various mineralogical or geognostical writings of the time. The first thorough examination of it as a relic of ancient volcanic action was that of Charles Maclaren, published in 1839.[95] This author clearly recognized the later age and unconformable position of the coarse mass of agglomerate pierced by the basalt of the apex, and pointed out the evidence of the upheaval and denudation of the older volcanic series during a long interval of repose before the latest eruptions took place. Subsequently Edward Forbes suggested that the upper part of the hill might be of Tertiary age.[96] Thereafter I mapped the ground in detail for the Geological Survey, entirely confirming the observations of Maclaren.[97] In the end it seemed to me that the interval between the two epochs of volcanic activity might not be so great as Forbes had supposed; and after tracing the Permian vents of Ayrshire, I came to the conclusion that the younger unconformable agglomerate of Arthur Seat was not improbably Permian.
[95] Geology of Fife and the Lothians, p. 34. In a reprint of this work, published in 1866, the venerable author briefly remarked in a footnote that he no longer believed in the second period of volcanic activity. This view was adopted in 1875 by Professor Judd, Quart. Journ. Geol. Soc. xxxi. p. 131. For the reasons stated in the text I believe Maclaren's original explanation of the structure of the hill to be correct.
[96] Forbes never published his views regarding Arthur Seat, but expounded them to his class, and explained them in diagrams, some of which are preserved in the Edinburgh Museum of Science and Art, in association with the specimens which he collected from the hill.
[97] Sheet 32, Geol. Survey of Scotland and descriptive Memoir. See also Rep. Brit. Assoc. 1867, address Geol. Sect., and Murchison's Siluria, 4th edit. p. 331.
The older volcanic series of this hill has been broken through by the agglomerate which occupies a true neck, and is abruptly marked off from all the rocks older than itself. There is no trace of any of the older lavas or tuffs thickening towards this vent. On the contrary they are completely truncated by it, and their outcrops on the north side reappear from under the agglomerate on the south side. Their escarpments are wrapped round by the agglomerate which likewise fills the head of the hollow that had been previously worn by denudation out of the stratified deposits between the oldest lavas. There is thus a violent unconformability between the later and the older volcanic rocks of Arthur Seat.
The length of time indicated by this stratigraphical break must be great. There is no known discordance in the Carboniferous system of the Lothians, yet the Coal-measures, Millstone Grit, Carboniferous Limestone series and much of the Calciferous Sandstones were stripped from this hill before the eruption of the agglomerate. It will be shown in the sequel that a nearly similar amount of denudation preceded some of the probably Permian eruptions of Fife.
The agglomerate contains abundant fragments of the older volcanic series. Its matrix is a dull red gravelly detritus, crowded with blocks of all sizes up to a yard or more in diameter. It is pierced by a column or plug of basalt, which sends veins into it, and rises to the apex of the hill. A beautiful olivine-basalt forms the lateral mass of the Lion's Haunch, which rests on the agglomerate.
Fig. 207.—Section showing the relations of the later rocks of Arthur Seat.
1. Grey and reddish sandstones and shales (Calciferous Sandstones); 2. The lava of the Long Row: the oldest of the Carboniferous volcanic series; 3. Tuffs of the Dry Dam; 4. Columnar basalts overlying the tuffs; 5. Andesite lavas of the eastern half of Arthur Seat; 6. Sill of Heriot Mount; 7. Sill of Salisbury Crags; 8. Sill of the Dasses. These complete the Lower Carboniferous volcanic series (compare [Fig. 112]). 9. White sandstones and black shales, upper division of the Calciferous Sandstones; 10. Younger volcanic agglomerate resting on the denuded ends of the older volcanic series; 11. Basalt of the summit sending veins into the agglomerate; 12. Basalt of the Lion's Haunch.