CHAP. XX 
ERUPTIONS OF THE PENTLAND HILLS 
333 
the uppermost group of andesites, which, as before, ai’e here truncated by 
the eastern fault (/), the Caleiferous Sandstones and Carboniferous Lime- 
stone series (6) being thrown against them. 
A third section (Lig. 89), taken two miles still further south, shows a 
remarkable attenuation of the volcanic series, and the appearance of a thick 
group of conglomerates (2) lying conformably below that series, hut resting 
on the upturned edges of the upper Silurian shales (1). The thick Aller- 
muir porphyrites are here reduced to a few thin beds (3) intercalated among 
the conglomerates and sandstones, amidst which the whole volcanic series 
dies out southward. A detailed section of the rocks exposed on the western 
front of Braid Law shows the following succession : — 
Wliite felsitio rocks of Braid Law (4 in Fig. 89). 
Coarse conglomerate passing down into sandstone. About 20 feet visible. 
Dark andesite, 4 feet. 
Parting of yellow felspatliic grit, 8 or 10 inches. 
Andesite, 10 feet. 
Hard felspatliic grit, 0 feet. 
Dark green aniygdaloidal andesite, 2 feet. 
Yellow felspatliic sandstone and grit, 2 feet. 
Dark green aniygdaloidal andesite, 6 feet. 
Felspatliic grit and red and brown sandstone, 4 feet. 
Dark andesite, perhaps 6 or 8 feet. 
Great conglomerate with alternating courses of sandstone, rapidly increasing in 
thickness southwards. 
Above these dwindling Representatives of the northern andesitic lavas 
comes the continuation of the white band of tuffs and breccias of Caer- 
ketton and Scald Law (4), which in turn dips under the highest group of 
andesites. The Carboniferous strata (5) are brought in by the fault (/). 
In little more than two miles beyond this line of section the volcanic 
series disappears, and the Old Bed Sandstone for a brief space consists only 
of sedimentary deposits. 
Besides the remarkable alternation of basic and acid ejections, there is a 
further notable feature in the geology of the Bentland Hills. This volcanic 
centre presents us with one of the most remarkable vents anywhere to be 
seen among the volcanic rocks of Britain. The full significance of this 
feature may best be perceived if we advance along the hills from their 
south-western end. As has now been made clear, the volcanic materials 
which begin about the line of the North Esk near Carlops rapidly augment 
in thickness until, in a distance of not more than seven miles, they attain a 
thickness of about 1 000 feet, and then form the great scarped front of the 
hills that look over Edinburgh. But at the base of tliat wall their con- 
tinuity abruptly ceases. The lower ground, which extends thence to the 
southern suburbs of Edinburgh, and includes the group of the Braid Hills, is 
occupied by another and more complex group of rocks in which the paral- 
lelism and pei’sistence so marked in the Bentland chain entii'ely disappear. 
This abrupt truncation of the bedded lavas and tuffs marks approximately 
the southern margin of a large vent from which at least some, if not most. 
