282 
VOLCANOES OF THE LOWER OLD RED SANDSl'ONE 
BOOK V 
interstratificatious, their differences of texture and consequent variations in 
mode and amount of weathering usually suffice to mark them off from each 
other, and to indicate their trend along the surface in successive terraces. 
Even where the angles of inclination are high, the bedded arrangement can 
generally he detected. 
It is in the picturesque and instructive coast-sections, however, that the 
details of this bedded structure are most clearly di.splayed. On both sides 
ot the country, along the shores of Ayrshire on the west, and those 
ot Kincardineshire and Forfarshire on the east, the volcanic group has 
heeii admirably dissected by the waves. The lava-beds have heen cut in 
vertical section, so that their structure and their mode of superposition, one 
over another, can be conveniently studied, while at the same time, the upper 
surfaces of many of the flows have been once more laid bare as they existed 
before they were buried under the sedimentary accumulations of the waters 
in which they were erupted. 
Though distinctly bedded, the Lavas show little of the regularity and 
persistence so characteristic of those of Carboniferous and of Tertiary time, 
yoine of them are not more than from four to ten feet thick, and generally, 
on the coast-cliffs, tliey appear to be less than fifty feet. A continuous’ 
gioup of sheets can sometimes be traced for ten miles or more from the 
probalile vent of discharge. 
That many of these lavas were erupted in a markedly pasty condition 
may lie inferred from certain of their more prominent characteristics. 
Sometimes, indeed, they appear as tolerably dense homogeneous masses, 
breaking with a kind of prismatic jointing ; but more frequently they are 
strongly amygdaloidal, and sometimes so much so tliat, as already stated, 
the amygdales form the larger proportion of their bulk. Where the secondary 
mfiltration-products have weathered out, the rough scoriform rock looks as 
if It might only recently have been erupted. In a few instances I have 
observed an undulating rope-like surl'aee, which reminded me of well-known 
Vesuvian lavas. Usually the top and liottom of each sheet assume a 
strikingly slaggy aspect, which here and there is exaggerated to sucli an 
extent that lietween the more solid and homogeneou.s parts of two consecutive 
flows an intermediate liand occurs, ten or twelve feet thick, made up of 
clinker-like lumps of slag, the interspaces being filled in with hardened 
sand. In some cases these agglomeratic layers may actually consist in part 
of ejected blocks ; but the way in whicli many of the lavas have cooled in 
rugged scoriaceous surfaces is as conspicuous as on any modem couUe. The 
loosened slags, or the hroken-up cakes ami l.locks of lava, have sometimes 
been caught up in the still moving, pasty current, which has congealed 
with its vesicles drawn out round the enclosed fragments, giving rise to a 
mass that might be taken for a breccia or agglomerate. Kow and then we 
may observe that the upper slaggy portion of a sheet has assumed a bri^lit 
red colour from the oxidation of its ferruginous minerals ; and from the 
contrast it thus presents to the rest of the I’ock we may jierhaps legitimately 
infer that the disintegration took place before the outflow of the next 
