VOLCANOES OF THE LOWER OLD RED SANDSTONE book v 
318 
renewed study a wider knowledge of the subject. A new edition of the 
map was issued in 1892, and I shall here embody in my summary the 
chief results obtained in the course of this revision. 
The most obvioir.s features in the Pentland area are the marked develop- 
ment of the volcanic rocks at the north end of the chain, their rapid 
diminution and disappearance towards the south-west, the abrupt truncation 
of the bedded masses by the line of craggy declivity which forms the 
northern termination of the hills, and lastly, the continuation of the volcanic 
series northward in a totally different form in the lower eminences of the 
Braid Hills. 
The length of the whole volcanic tract is about eleven miles ; its breadth 
at the widest northern part is four miles, but from that maximum it dwindles 
southwards and dies out in seven miles. Its western side is in large measure 
flanked by the unconformable overlap of the Upper Old Eed Sandstone and 
Lower Carboniferous formations, though in some places the base of the 
volcanic series is seen. The eastern boundary is chiefly formed by a large 
fault which brings down the Carboidferous rocks against the volcanic ridge. 
At the northern end, this ridge plunges unconformably under the Upper Old 
Bed Sandstone of the southern outskirts of Edinburgh. 
The bedded aspect of the truncated end of the Pentland chain, as seen 
from the north, has been already alluded to (p. 281). The rocks dip to the 
south-east, hence the lower members of the series are to be found along the 
north-west side of the hills. 
It will be noticed from the Geological Survey map that the volcanic 
rocks of the main body of the Pentland Hills are arranged in alternations of 
:fillermui rJiUi Caerkettort Kill 
Fig. 86. — Suction across tlie north uinl of the Peiitlainl Hills, from Warklaw Hill to Pentland Mains. 
Length about five miles. 
1 . Upper Silurian grits and shales, not seen where the line of section crosses; 2 2. Andesites and diabases in 
mnneroiis interstratified sheets ; 2 s. Intercalated sandstones and conglomerates ; 3. Felsitic tutts and 
breccias and ortliophyre sheets ; n, Volcanic neck ; 4. Lower Carboniferous stiuta lying unconformably on and 
overlapping the volcanic series; 5. Calcifcrous Sandstones and Carboniferous I.iine.stone series brought down 
against tlie volcanic series by a fault (/). 
.somewhat ba.sic and more acid bauds. The most basic sheet.s are some 
amygtlaloidal diabases at the bottom of the whole series which make their 
appearance in Warklaw Hill (Fig. 86). The greater number of the dark 
lavas are varieties of andesite, sometimes tolerably compact, sometimes 
highly cellular and amygdaloidal. But interstratified with these are thick 
sheets of wliat used to be called “ claystone,” a term which here comprised 
decayed felsites (orthophyres), and also felsitic tuffs and breccias. The re- 
markably acid nature of some of these rocks has been already pointed out. 
The total thickness of the volcanic series at the north end of the hills is 
about 7000 feet, but as neither the top nor the bottom is there visible, it 
