320 
VOLCANOES OF THE LOWER OLD RED SANDSTONE book v 
along the upper and under parts of tlie several sheets. No tuff has been 
noticed between these basic Hows, but here and there thin lenticular layers 
of sandstone, lying in hollows of the lava-sheets, are connected with vertical 
or highly-inclined ramifying veins of similar material, with the jilains of 
stratification passing across the breadth of the veins. These features are an 
exact reproduction of those above described in Forfarshire and Kincardine- 
shire. The aniygdales consist of chalcedony, crystallized quartz and calcite. 
Torduff Hill, which rises to tlie east of Warklaw, consists of a mass of 
coarse volcanic breccia or agglomerate (m), 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 tliem are to be found. The hill is covered with 
grass, but among the rubbisli 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 anotlier liand of pale material forms the conspicuous 
craggy face so familiar in the aspect of the Pentland Hills as seen from 
Edinburgh. This baud 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. Tlie 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 
liand of rock is exposed in a few crags and quarries, and its debris, pro- 
truding 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 How-structure of rhyolite. 
The breccia which projects in blocks from the summit of the liill has been 
quarried immediately below the crest on tlie south side, where it overlies 
a thin intercalated baud 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 
