304 
VOLCANOES OF THE LOWER OI.D RED SANDSTONE book v 
the lavas range continuously for nearly fifty miles to the north-east, until 
they reach the sea at Tayport ; hut they are prolonged on the north side of 
the Firth of Tay from Broughty Ferry to near Arhroath, so as to overlap those 
of the Montrose group. Tliey thus attain a total length of nearly sixty 
miles in a north-easterly line. How far they stretched south-west cannot 
now he ascertained, for they have been dislocated and buried in that direc- 
tion under the Carboniferous formations of the Midland Valley. 
It will be obseiued from the map (Xo. 111.) that the great volcanic 
ridge of the Ochil Hills continues unbroken for twenty-two miles, from 
Stirling to Bridge of Earn. Thereafter it branches into two divergent portions, 
one of which runs on through the north of Fife to the southern promontory 
of the estuary of the Tay, wliile the other, after sinking below the alluvial 
plains of the Earn and the Tay, mounts once more into a higli ridge near 
Perth, and thence stretches eastward into Forfarshire as the chain of the 
Sidlaw Hills. This bifurcation is due to the opening out and denudation of 
the great anticlinal fold above mentioned. The rocks in the northern linilj 
dip north-westward, those in the southern limb dip south-eastward. The 
lower members of the Old Bed Sandstone, underlying the volcanic series, 
ought to be seen beneath them along the crest of the anticline. Unfor- 
tunately, however, partly by the action of faults along the boundaries of the 
volcanic bands, but chiefly from the unconformable overspread of Upper Old 
Bed Sandstone and Lower Carboniferous rocks across the plains of the Cai'se 
of Cowrie and of the Earn, the lower parts of t)ie system are there concealed 
(see Fig. 78). As already remarked, this important anticlinal fold runs to 
the north-east across Forfarshire, and passes out to sea nortli of Montrose. 
Through the Ochil chain tlie fold runs oblifpiely in a soutli-westerly 
direction, until it is truncated by the great fault which lets down the 
Clackmannan coalfield. The total traceable length of this anticline is thus 
about sixty miles. It flattens down towards the south-west ; consequently 
the rocks in the western part of the Ochil Hills are so gently inclined that 
the same bands may be followed winding round the sides of the valleys, and 
giving to the steep declivities the terraced contours to which allusion has 
already been made (see Fig. 68). Another result of this structure is that 
the base of the volcanic series is entirely concealed by its higher portions. 
From an examination of the map it will be further obvious that the 
whole wide plain of Strathmore — that is the great hollow, more than 8,0 
miles long and about ten or twelve miles broad, which stretches between the 
base of the Highland mountains and the north-western slopes of the Ochil 
and (Sidlaw chain — is underlain with volcanic rocks of Lower Old Bed Sand- 
stone age. This plain lies on a broad synclinal fold, along the south-east 
side of which the lavas, tuffs and conglomerates of the Ochil and Sidlaw 
Hills dip under a thick accumulation of red sandstone and flagstone. On 
the north-west side similar lavas and tuffs rise again to the surface, both on 
the southern side of the great boundary faults, and also in the little bays 
which liere and there survive on the northern side of the dislocations 
(Fig. 77). 1 have already alluded to these interesting relics of the shore- 
