288 
VOLCANOES OF THE LOWER OLD RED SANDSTONE kook v 
periods. This deficiency doubtless arises from the geologictil structure of 
the districts in which the formation is chiefly developed. Thus, in the 
great Midland Valley of S(;otland, where the Old Red Sandstone covers a 
large part of the surface, the vents seem to have been placed along the 
central parts of the long trough rather than among the older rocks on either 
margin. Hence they are in large measure buried either under the volcanic 
and sedimentary accumulations of their own period or under Carboniferous 
strata. 
Certain bosses of massive rocks lying well within the volcanic area 
may with some confidence be regarded as the sites of eruptive centres. 
They occur either singly or in groups, and may be specially noticed along 
the chain of the Oehil and Sidlaw Hills. Yet it seems to me probable 
that these visible bosses, even if we are correct in regarding them as 
marking the positions of true vents, do not indicate the chief orifices of 
discharge. If we consider their size and their distribution with reference 
to the areas of lava and tuff discharged at the surface, we are rather led to 
Fig. 68. — Section across two necks above Tillicoultry, Ochil Hills. 
1 1, Andesite lavas ; 2 2, Tuffs and volcanic conglomerates ; 3 3, The two necks ; 4 4, Dykes of felsite, etc. ; 
5, Coal-ineusuros ; /, Fault. 
look upon them as subsidiary vents, the more important orifices, from which 
the main bulk of the eruptions took place, being still concealed under the 
Carboniferous rocks of the Midland Valley. The bosses which rise through 
different portions of the volcanic series are obviously not the oldest or original 
vents. The great felsitic mass of Tinto in Lanarkshire (Fig. 93), indeed, 
pierces strata which lie near the base of the Lower Old Red Sandstone, but 
the smaller cone of Quothcpian in its neighbourhood appears in the midst of 
the lavas (Fig. 92). In the south-western part of the Ochil cliain the bosses 
or necks are chiefly .small in size, seldom exceeding half a mile in diameter. 
They have been filled sometimes with crystalline, sometimes with fragmental 
materials. Two of them, containing the remarkable granophyric quartz- 
diorite already referred to, emerge from among the tuffs in a low part of 
the .series, immediately above the village of Tillicoultry in Clackmannan 
(Fig. 68). Two or three more, which are occupied by orthophyres and 
(;[uartz-felsites, pierce the volcanic group a few miles to the west of Locli 
Leven. The whole of the visible bosses of the Ochil Hills may be regarded 
as one connected group), subsidiary to the main orifices which lay rather 
