(1) _Vents._--On the south side of the estuary of the Forth there is only one neck which may be plausibly placed in this series. It forms the upper part of Arthur Seat, at Edinburgh. This hill has already been cited as consisting of two distinct portions. The lower, built up of bedded tuffs, basalts and andesites, forms part of the Midlothian volcanic plateau of Carboniferous time. The vent from which these materials were ejected must lie at some little distance, and its site has not been certainly ascertained. The upper part of the hill is formed of a distinct group of rocks which has now to be described. 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. [Footnote 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.] [Footnote 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.] [Footnote 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.]