CHAP. XXXI 
THE BASIN OF THE FIRTH OF FORTH 
67 
display of similar intrusive sheets. The best district for the study of these 
sills lies around Dalmellington. The Coal-measures are there traversed by 
many intrusions, which have produced great destruction among, the coal- 
seams. Some of the rocks are extremely basic, including a beautiful picrite 
like that of Incheolm (Letham Hill, near Waterside). The age of these 
sills must he later than the Coal-measures into which they have been 
injected. Some of them are obviously connected with the agglomerate-necks, 
and the whole or the greater number should thus probably be assigned to 
the Permian period. 1 The phenomena of intrusion presented by these rocks 
reproduce the appearances already described in connection with the basic 
intrusive sheets of Carboniferous age. 
2 . Basin of the Pirth of Forth 
The other district of Southern Scotland, where traces ol volcanic action 
later in age than the Coal-measures may be observed, lies in the basin ot 
the Pirth of Forth (Map V.). They include no bedded lavas, and only 
at one locality do any relics of a covering of stratified tuffs overspread the 
Carboniferous formations. The evidence for the old volcanoes consists 
almost entirely of necks of tuff, which mark the position of vents of eruption. 
( 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 Car- 
boniferous -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 Xeptunist and Plutonist contro- 
versy, 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 .1.8 .j 9 . 
This author clearly recognized the later age and unconformable position of 
the’ coarse mass of agglomerate pierced by the basalt of the apex, am 
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. 3 Thereafter I mapped the ground 111 
1 Explanation of Sheet 11, Geol. Snrv. Scotland, P- 22. , 
2 Geology of Fife and the Lothians, p. 34. In a reprint of this work, published m 1866 , the vene- 
able author briefly remarked in a footnote that he no longer believed in the second period of. volcanic 
activity. Tins view was adopted in 1875 by Professor Judd, Quart. Jour*. Geol Soe. P- m • 
For the reasons stated in the text I believe Maclaren’s original explanation of the structure 
hill to be correct. , . , ..l,,..,, 
2 Forbes never published his views regarding Arthur Seat, but expounded them to hi , 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 In . 
