310 
VOLCANOES OF THE LOWER OLD RED SANDSTONE book v 
spicuous feature. The whole of that hill consists of a constant alternation 
of lavas (chiefly slaggj" andesites, but including also one felsitic how) with 
hands of coarse and finer tuff and volcanic conglomerate. The greatest 
continuous mass of this fragmental material is (500 or 700 feet thick. 
From the extraordinary size of its included blocks it obviously imrst have 
been formed of ashes, stones and huge pieces of lava ejected from some 
vent in the near neighbourhood. Some of the individual blocks in this 
mass are as large as a Highland crofter’s cottage. 
The uppermost lavas of Dumyat dip under a still higher series of coarse 
volcanic conglomerates entirely made up of andesitic debris and reaching a 
thickness of about 1000 feet. This enormous accumulation was probably 
due partly to the abrasion of exposed cones and lava-ridges, and partly to 
volcanic discharges of fragmentary materials. Yet it is worthy of note that 
even amidst tliese evidences of the most vigorous volcanic activity we have 
also proofs of quiet sedimentation and traces of the fishes that lived in the 
waters of the lake. This particidar zone of coarse conglomerate as it 
extends in a south-westerly direction becomes finer, and its upper part 
passes into a chocolate-coloured sandstone which has been quarried at 
Wolfe’s Hole, Westerton, Bridge of Allan, at a distance of about three 
miles from where the line of section runs, which is embodied in the diagram. 
Fig. 81. It was from this locality that the specimens of Eucephalasins, 
Ptemspis and Scapliaspiis were obtained which were described by Professor 
Piay Lankester.^ 
Alrove the last-named thick group of coarse volcanic conglomerates a 
solitary sheet of dark slaggy andesite may be observed. This lava is then 
overlain by the great depth of chocolate-coloured and red sandstones and 
marls of the plain of Strathmore (c in Fig. 81). Nevertheless a few 
hundred feet up in these sedimentary deposits we meet with yet one 
further thin sheet of lava — the last known eruption of the long volcanic 
history of this district. 
Before quitting the Ochil range I may refer to the evidence there 
obtainable as to the horizontal extent of separate sheets of lava. The 
western end of this range affords great facilities for following out individual 
lieds of andesite along the Imre terraced front of the great escarpment. 
Thus, the easily recognizable porphyrite which caps King’s Seat Hill, above 
Tillicoultry (see Fig. 68), can be traced winding along the hill-slopes until 
it descends to the plain, and is then lost under the great fault, at the foot 
of Dumyat — a distance of more than six miles. There is, therefore, no 
difficulty in supposing that from the Ochil line of vents streams of lava 
should have rolled along the iloor of the lake across to the base of the 
Highland slopes, 10 or 12 miles distant. We cannot tell, of course, 
whether any buried vents lie below the plain of Strathmore, but certainly 
no unquestionable trace of vents has yet been found among the crystalline 
rocks along the borders of the Highlands.'^ 
1 Pala'O'iUoc/raphical Society, voLs. xxi. (1867) and xxiii. (1869). 
- Allusion has already been made to the possible connection of the younger Highland granites 
