CHAP. XX 
ERUPTIONS OF THE PENTLAND HILLS 
321 
several indies long. They not infrequently display a line and e.xtreniely 
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 those highly-siliceous tufis. Unfortun- 
ately the manner in which the rocks decay and conceal themselves under 
their own debris makes it difficult to separate the undoulitedly fragmental 
bands from those which may be true lavas. But an occasional opening, 
and here and there a scattei’od 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 sciile. The bands of varied devitrification are some- 
times 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 decep- 
tive layers of diflerent 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. Tliere 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 ot 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 Bentland Hills, it will be found to expose the 
arrangement of rocks represented in Fig. 88. At the western end the 
Upper Old Bed 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 Bed 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 con- 
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VOL. I 
