322 
VOLCANOES OE THE LOWER OLD RED SANDSTONE book v 
glomerate. 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. 
To the south of the Silurian shales that lie against the southern flank 
of North Black Hill, pale felsitic tuffs (3) occur, which are a continuation of 
those already referred to as running southwards from Capelaw Hill. Above 
them a series of andesites (2), with intercalated bands of tuff, sandstone 
and conglomerate (2 s), occupy the bottom of the Logan valley and part 
of the slopes on both sides. In the thickest band of tuff's, which is well- 
Fig. 88. — Section across the Pentlaud Hills through North Black Hill and Scahl Law (length about 
three miles). 
exposed along the road by the side of the Loganlee Eeservoir, a group of 
well-bedded strata occurs from less than an inch to a foot or more in thick- 
ness. Generally they are pale in colour, and are made up of white felsitic 
detritus, but with a sprinlcling of dull purplish-red fragments, and occasional 
larger rounded pieces of different andesites. Some of the rocks might be 
called felspathic sandstones. Other bands in the group are dark purplish- 
red in tint, and consist mainly of andesitic debris, with a dusting of white 
felsitic grains and fragments. There would thus seem to have been showers 
both of felsitic and of andesitic ashes and lapilli. 
The dark lavas that overlie the tuff's are likewise well displayed along 
the same road-section. They vary rapidly from extremely compact homo- 
geneous dark blue rocks, that weather with a greenish crust, to coarse, 
slaggy masses and amygdaloids. 
These more basic lavas are a continuation of those of Allermuir Hill, 
and, as at that locality, they plunge here also under the same band of white 
tuffs, breccias and felsites (3), which has been referred to as stretching 
Green La.^ 
^raidZaw 
Fig. 89.— Section from tlie valley of tlie Gutterford Burn through Green Law and Braid Law to 
Eight- Mile Burn. 
southward from Caerketton Crags. This band must here be at least 500 feet 
thick. It forms Scald Law (1808 feet) and the surrounding summits, and 
thus occu})ies the highest elevations in the Pentland chain. It dips beneath 
