CHAP. XIX 
THE OCHIL AND SIDLA IV ERUPTIONS 
305 
line of Lake Caledonia, and to the fact that though they lie unconforinahly 
on the Hifdiland schists, they do not belong to the actual basement members 
of the Old” Red Sandstone {wnte, p. 295, and Fig. 73). We have seen that 
Ijelow the bottom of the volcanic series a thickness of 5000 feet of sand- 
stones and shales emerges on the Stonehaven coast, and yet that even there 
the base of the whole system is not visible, owing to the effect of the High- 
land boundary fault. . .v,- 1 . 
It is thus evident that over the bottom of Lake Caledonia a very thick 
deposit of tolerably fine sedimentary material was spread before the commence- 
ment of the Ochil and Sidlaw eruptions, — that when the lavas were poured 
out and the coarse conglomerates began to be formed, these materials over- 
lapped the older deposits and gradually encroached upon the subsiding area 
Fig. 77.-Section acro.^s the Boimdary-lault of the Highlands at Glen Turrit, Perthshire, 
s, crystalline schists of the Old Red Sandstone) .ith inter- 
of the Hio'hlands. The lavas rolled across the floor of the lake and entered 
the successive bays of the northern coast-line, where tlieir outlying patches 
may still be seen. e i • 
From these facts it is clear that to the actually visible area of volcanic 
material in the Ochil and Sidlaw region, and to the anticlinal tract whence 
the andesites have been removed by denudation, we have to add the area 
that lies under the plain of Strathmore, which may be computed to be at 
least 800 square miles, making a total of jirobably not less than loOO 
square miles. But it will be remembered that practically only one side of 
the anticlinal fold is accessible to observation. We cannot tell how far in a 
southerly direction the lavas of the Ochil Hills may extend. It is quite 
possible that not a half of the total area covered by the eruptions of this 
volcanic group is now witliin reach, either of observation or of well-founded 
inference. . . . 
One further general characteristic of this volcanic district will be obvious 
from an inspection of the map. While the thickest mass of lavas and tuffs, 
lyim. towards the soutli-west, points to the existence of the most active vents 
in tLt part of the area, the actual positions of these vents have not been 
detected. Probably they lie somewhere to the south of the edge of the 
Ochil chain, under the tract which is overspread with the coalfield, but 
other and possibly minor orifices of eruption appear to liave risen at 
irregular intervals towards the north-east along the length of the lake. Ihus 
there are numerous bosses of felsitic and andesitic rocks among the central 
Ochils, some of which may mark the positions of active \ents. For some 
miles to the east of that area an interval occurs, marked by the presence of 
VOL. I 
