182 Proceedings of Royal Society of Edinburgh, [april 4 , 
(1) The Basement Conglomerate . — The conglomerate of Bell 
Hill would thus appear to be the oldest member of the Old Red 
Sandstone series. It consists principally of water-worn shingle and 
gravel, set in a matrix of arenaceous and argillaceous matter. The 
stones, as mentioned already, have all been derived from the con- 
tiguous Silurian strata. The lower appear to he upon the whole 
coarser than the upper beds, stones 6 inches and more in diameter 
being common in the former, while the latter are rather conglomer- 
atic and pebbly grit and sandstone than conglomerate. The beds 
dip towards the south-east, hut are turned up against the N.E. and 
S.W. fault, while they trend steeply down towards the N.W. and 
S.E. fault. The series is probably about 100 feet in thickness. 
(2) The Igneous Series. — The lowest beds seen on the north 
side of the N.W. and S.E. fault is a bedded porphyrite which is 
overlaid by a thick layer of agglomeratic tuff. Above this comes a 
succession of bedded porphyrites, about 250 feet or so in thickness, 
and these beds are succeeded by 40 to 50 feet of various tuffs, 
which dip in their turn under a second group of porphyrites. 
These last do not appear to exceed 250 or 300 feet in thickness, 
and are overlaid by some 400 feet of bedded tuffs. 
Bedded Porphyrites. — These rocks having all the same character, 
one general description will suffice. They are for the most part 
fine-grained, purplish-blue or greyish-blue in colour, but frequently 
stained brown or red with much diffused ferric and hydrous 
ferric oxide. The joint-faces especially are often coated with hae- 
matite and limonite, while thin veins and threads of the same 
minerals are common. The rocks do not differ in general appear- 
ance from the porphyrites of Old Red Sandstone and Lower 
Carboniferous age which occur elsewhere in Scotland. They are, 
upon the whole, not so markedly porphyritic with plagioclase as the 
porphyrites of other districts, but closely resemble such fine-grained 
rocks as that of Blackford Hill, and similar rocks met with in the 
Braids and Pentlands. They are often highly scoriaceous and 
amygdaloiclal above and below, and not infrequently contain, both 
in their upper and under portions, irregular areas of fine-grained 
tuff, consisting of amorphous, dust-like material, and comminuted 
debris and small lapilli of highly porous porphyrite (see fig. 4). 
In the upper parts of some of the old lava-flows this tuff appears 
