324 
VOLCANOES OF THE LOWER OLD RED SANDSTONE kook v 
of these rocks were probably ejected. The size of this vent cannot be pre- 
cisely ascertained on account of the unconforniable overspread of Lower 
Carboniferous strata. But that it must have been a large and important 
volcanic orifice may be inferred from the fact that tire visible area ol the 
materials that fill it up measures two miles from north-east to south-west, 
and a mile and a half from south-east to north-west, thus including a space 
of rather more than two square miles. Its original limits towards the north 
and south can be traced by help of the bedded lavas that partially surround 
it, but on the two other sides they are concealed by the younger formations. 
We shall probably not over-estimate the original area of the vent if we state 
it at about four square miles. 
The materials that now fill this imporant orifice consist mainly of “ clay- 
stones,” like those of the Pentland series — dull rocks, meagre to the touch, 
varying in texture from the rough porous aspect of a sinter through stages 
of increasing firmness till they become almost felsitic, and ranging in colour 
from a dark purple-red, through shades of lilac and yellow, to nearly white. 
Fig. 90. — Sectiou aci’oss the north entl of the Pentland Hills, and the soutliern edge of the Braid Hill 
vent. Length about two miles. 
1 1. Andesites ; 2. Fine tuUs, etc., of the Braid Hill vent ; 3 .S 3. Agglomerate in lateral necks with 
felsitic iutnisious (4). 
hut often strikingly mottled. A more or less laminar structure is often to 
be observed among them, indicating a dip in various directions (but especially 
towards the north) and at considerable iingles. Throughout this exceedingly 
fine-grained material, lines of small lapilli may occasionally he detected, also 
bands of breccia, consisting of broken-up tuff of the same character, and 
of fine “ hornstone ” and felsite, with delicate fiow-structure. Exhibiting 
on the whole so little structure, this tract may be regarded as consisting 
largely of fine volcanic dust derived from the explosion of felsitic or ortlio- 
phyric lavas. Some portions indeed are not improbably composed of 
decayed felsites, like those which present so many difiiculties to the geologist 
who would try to trace their course among the other lavas and tuffs of the 
Pentland chain. Various veins, dykes and small bosses of felsite, andesite 
and even more basic material, such as fine dolerite, have been intruded into- 
the general body of the mass. 
On the outskirts of the main vent some subordinate necks may be 
observed (3, 3 in Pig. 90), perhaps, like Torduff Hill, already noticed (Pig. 
86), marking lateral eruptions from the flanks of the great cone. Three 
of these occur in a line more than half a mile long, possibly indicating a 
fissure on the side of the old volcano, running in a soutli- westerly direction 
