Torduff Hill, which rises to the east of Warklaw, consists of a mass of coarse volcanic breccia or agglomerate (n), markedly felsitic in its materials. It probably forms a neck marking a small volcanic vent, like some others at the north end of the chain to be afterwards referred to.
In the lower part of Capelaw Hill, the next eminence in an easterly direction, bedded andesites, with an intercalated band of sandstone and conglomerate (2s), appear and pass under rocks of so decomposing a kind that no good sections of them are to be found. The hill is covered with grass, but among the rubbish of the screes pieces of felsite-like rocks and breccias may be observed. Some of these blocks show an alternation of layers of felsitic breccia with a fine felsite-like material which may be a tuff. These rocks, conspicuous by the light colours of their screes, alternate further up with other dark andesitic lavas, and run south-westward for about five miles.
Beyond Capelaw Hill, upon a band of these pale rocks, comes a thick group of sheets of dark andesite, which form the main mass of Allermuir Hill. They are well seen from the south side and likewise from the north, dipping towards the south-east at angles of from 35° to 40°, and weathering along the crest of the hills into a succession of scars and slopes which show the bedded character of the lavas.
At Caerketton Hill another band of pale material forms the conspicuous craggy face so familiar in the aspect of the Pentland Hills as seen from Edinburgh. This band consists of pale felsitic breccia, and amorphous, compact, much-decayed rock, regarding which it is difficult to decide whether it should be considered as a fine felsitic tuff, or as a decomposed felsite. The band is better seen when traced southwards. The light colour of its screes makes it easily followed by the eye even from a distance along the hill-tops and declivities.
On the next hill to the south-west, known as Castlelaw Hill, this pale band of rock is exposed in a few crags and quarries, and its debris, protruding through the scanty herbage, slips down the slopes. On its north side the screes display the same felsitic breccias and compact, decayed felsitic rocks, occasionally showing a structure like the flow-structure of rhyolite. The breccia which projects in blocks from the summit of the hill has been quarried immediately below the crest on the south side, where it overlies a thin intercalated band of a dull, much-decomposed porphyry.
The breccias are composed almost entirely of thoroughly acid rock-fragments, as may be judged from the percentage of silica shown to occur in them. These fragments vary from the finest lapilli up to angular pieces several inches long. They not infrequently display a fine and extremely beautiful flow-structure. It is thus quite certain that there are acid breccias intercalated among the more basic lavas of the northern Pentlands, and that among the constituents of these breccias are fragments of felsite or perhaps even lithoid rhyolite.
We may therefore be prepared to find that actual outflows of felsitic lava accompanied the discharge of these highly-siliceous tuffs. Unfortunately the manner in which the rocks decay and conceal themselves under their own debris makes it difficult to separate the undoubtedly fragmental bands from those which may be true lavas. But an occasional opening, and here and there a scattered loose block, serve to indicate that the two groups of rock certainly do coexist in this pale band, which can be followed through the chain for upwards of six miles until it is cut off by the eastern boundary fault.
At the south-west end of Castlelaw Hill, where a quarry has been opened above the Kirk Burn, blocks of felsite may be observed showing flow-structure on a large scale. The bands of varied devitrification are sometimes a quarter of an inch broad, and weather out in lighter and darker tints. Some of them have retained their felsitic texture better than others, which have become more thoroughly kaolinized. That these are not deceptive layers of different texture in fine tuffs is made quite clear by some characteristic rhyolitic structures. The bands are not quite parallel, but, on the contrary, are developed lenticularly, and may be observed to be occasionally puckered, and to be even bent back and folded over as in ordinary rhyolites. There is no contortion to be observed among the stratified tuffs of the hills. This irregularity in the layers is obviously original, and can only be due to the flow of a moving lava.
On the east side of Castlelaw Hill, as shown in [Fig. 86], dull reddish andesites overlie the pale belt of felsitic rocks. Their lower bands are marked by the presence of well-formed crystals of a dark green mica. Their central and higher portions consist of porphyrites of the prevalent type, both compact and vesicular. These lavas continue as far as any rock can be seen. Beyond the boundary fault, the Burdiehouse Limestone and oil-shales of the Lower Carboniferous series are met with, inclined at high angles against the hills. It is impossible to say how much of the volcanic series has here been removed from sight by the dislocation.
If now we move three miles further to the south-west and take a second section across the Pentland Hills, it will be found to expose the arrangement of rocks represented in [Fig. 88]. At the western end the Upper Old Red Sandstones (4) and Lower Carboniferous series (5) are seen lying unconformably on the upturned edges of the Upper Silurian shales (1). North Black Hill consists of a large intrusive sheet of pale felsite (F) that has broken through the Silurian strata and has in places thrust itself between them and the conglomerates of the Lower Old Red Sandstone which lie unconformably upon them. In the neighbouring Logan Burn, at the bottom of the Habbie's Howe Waterfall, the felsite can be seen injected into the conglomerate. The felsitic sill of North Black Hill runs for a mile and a half along the western base of the volcanic series, and has a breadth of about half a mile. It is the only important intrusive mass in the Pentland Hills.