Volcanic vent parasitic on St. Leonard's Sill

Abstract

Introduction During a Saturday afternoon excursion to Arthur's Seat in April of last year, 1921, I drew the attention of Fellows of this Society to a strange anomaly in the behaviour of the Girnal Craig basalt. It is well to recall that several geologists have contributed directly or indirectly to our knowledge of this intrusion, notably Hay Cunningham,1 Charles Maclaren, Archibald Geikie, Judd, Bonney, Goodchild, and Peach. The Geological Survey has published Dr Peach's researches on Arthur's Seat both as a chapter in The Geology of the Neighbourhood of Edinburgh (1910), and, separately, as a Description of Arthur's Seat Volcano (1911 and 1921). The following points may be taken from his account. 1. The Girnal Craig basalt is a sill intruded among sediments a little below the base of the Arthur's Seat volcanic pile, which latter accumulated in early Carboniferous times. 2. It either refuses to enter, or is cut off by, the great complex vent of Arthur's Seat. 3. It reappears north of this vent in the Dasses, and west of the same, repeated by faulting, in St Leonard's Craig. Thus it has a minimum horizontal extension of about a square mile. 4. It is definitely linked by its petrology with the Arthur's Seat volcano. Accordingly, we may conclude that it entered partially consolidated sediment under a cover not much more (perhaps much less) than 2000 feet in thickness. The anomaly, referred to above, concerns the relations of the Girnal Craig basalt to the sediments immediately