CHAP. XX 
ERUPTIONS OF THE PENTLAND HILLS 
319 
may be considerably greater. At these maximum dimensions the rocks form 
the high scarped front of tlie Pentlaud Hills, which rises into so prominent 
a feature in the southern landscape of Edinburgh. A series of transverse 
sections across the chain from north to south will illustrate its structure 
and history. These I shall here describe, reserving for subseciueut con- 
sideration tlie great vent of the Braid Hills. 
A section taken through the north end of the chain, where the maxi- 
mum depth of volcanic material is exposed, presents the arrangement 
represented in Fig. 86. It will be seen that the base of the series is here 
concealed by the unconformable overlap of the Lower Carboniferous rocks 
on the west side, while the top is cut off by the great fault which on the 
east side brings down the Midlothian Coalfield. 
The Lower Carboniferous conglomerates (4) creep over the edge and up the 
slopes of the volcanic series of the Pentland Hills. They contain abundant 
Fig. 87.— View of tlie laYa-escarpraeuts of Warklnw Hill, Pentland clmiii, from the north-west. 
pebbles of the lavas, and were evidently laid down along a shore from which 
the Pentland rocks rose steeply into laud. Thougli tlie actual base of the 
lavas is not seen here, two miles further to the south highly-inclined 
Upper Silurian shales and mudstones are found emerging uncoiiformably 
from under the volcanic pile, and similar strata probably underlie Warklaw 
Hill as indicated in the figure. The Upper Silurian strata pass up into a 
lower group of the Lower Old Pied Samlstone, which has also been covered 
uncoiiformably by the volcanic series. In these underlying deposits we 
have evidence of the pre-volcanic accumulations of the lake, which were 
broken up and tilted at the beginning of the volcanic eruptions. 
The lowest lavas, consisting of well-marked beds of diabase (2), present 
their escarpments to the north-west and dip into the rising ground, as 
sketched in Fig. 87. Their characters have been already noticed in the 
general petrography of the Old Eed Sandstone volcanic rocks. Dark solid 
compact portions of them pass rapidly into coarsely cellular slag, especially 
