Cairn Table (i94-ZFi) Jlirkleac (J535Fi) 
330 
VOLCANOES OF THE LOWER OLD RED SANDSTONE book v 
M 
The volcanic series in this limited district reaches an estimated thick- 
ness of 4000 feet, built up of purple and green 
slaggy andesites, dark heavy diabases (melaphyres) 
and tuffs, with abundant interstratification of sand- 
stone, especially towards the base. One of its chief 
features of interest is the manner in which it 
exhibits, better, perhaps, than can be found in any 
of the other volcanic areas, the fretprent and rapid 
alternations of lavas and tuffs with sandstone and 
conglomerate. In this part of the region the 
volcanic discharges were obviously frequent and 
intermittent, w’hile at the same time the transport 
and deposition of sediment were continuous. This 
sediment consisted largely, indeed, of volcanic de- 
tritus mixed with ordinary sand and silt. That 
these conditions of sedimentation were not wholly 
inimical to animal life is shown by the occasional 
occurrence of worm-burrows in the ashy sandstones.^ 
The thick accumulation of sandstones and con- 
glomerates above the main mass of lavas has been 
derived almost wholly from the waste of the vol- 
canic rocks (3). Blocks of andesite, w'ell rounded 
and often from six to twelve indies in diameter, may 
be seen in the remarkable band of coarse con- 
glomerate whicli runs as a nearly continuous ridge 
from the Mth to the Clyde — a distance of more 
than twenty miles. Nothing impresses the geologist 
more, as he wanders over this district, than the 
evidence of the prodigious waste which the volcanic 
series underwent before it was finally buried. Some 
part of the detritus may have been supplied, in- 
deed, by occasional discharges of fragmental matter, 
as has already been suggested in the case of the 
Ochil and Montrose conglomerates. But the nature 
of the pebbles in these masses of ancient shingle 
g shows them to he not bombs, but pieces worn away 
5 from sheets of lava. 
That the lavas underlie these piles of detritus 
I and extend southwards even up to the very edge 
* of the Silurian Uplands is shown by the rise of a 
= number of successive beds from under the trough 
^ into which the conglomerate has been throwm. 
These lavas, hower'er, are almost immediately cut 
off by the great boundary fault (/) which flanks the 
That they are not met with now' to the south-east of 
C J£ 
5 £ 
’A 
s 
^ P3 
I s 
Silurian territory. 
' Memoir on Sheet 15 Geol. Suit. Scotland (1871), p. 22. 
