300 
VOLCANOES OF THE LOWER OI.D RED SANDSTONE book v 
bold promontory of the lied Head. There, at the base of the cliffs of red 
sandstone, the accompanying section may be seen. Beneath the red false- 
bedded and sometimes pebbly sandstones (c), which form nearly the whole 
precipice, lies a band of dull pnrplish ashy conglomerate (rf), composed 
almost wholly of fragments 
of different andesites, im- 
bedded in a paste of the 
same comminuted material. 
Towards the south, this 
rock rapidly becomes 
coarser, until it passes into 
a kind of agglomerate, in 
which the andesite blocks 
are sometimes a yard or 
more in diameter. It in- 
cludes bands of sandstone, 
which increase in number 
and thickness towards the 
north, and sometimes inter- 
vene underneath the con- 
glomerate. Tlie lowest rocks here visilde are sheets of andesite or “ por- 
pliyrite ” (a), separated from each other by irregular bright red layers of 
tufaceous sand and agglomei’ate. These lavas are dull purplish-grey to 
green, some of them being tolerably compact, others highly amygdaloidal, 
with large steam-cavities often drawn out in the direction of flow. 
One of the most striking features in the andesites of this coast is the 
remarkable manner in wliieh they include the veinings of pale green and red 
sandstone already descrihed (see Bigs. G5, GG). Some of tlie sheets liave in 
cooling cracked into rude polygcjiis. I’hey are likewise tra\'ersed by large 
cavernous spaces and intricate fissures or steam-cavities. Into all these 
openings tlie sand has been washed, filling them up and solidifying into well- 
stratified sandstone, the bedding of which is geneially parallel with that 
of the rocks that enclose it, the dip of the wliole series of strata being 
gently seawards. But a still more intimate mixture of the sand with the 
lava-sheets is to be remarked where these rocks assume their most slaggy 
character. In some of them the upper part, to a depth of ten or twelve feet, 
consists of mere rugged lumps of slag which, while the mass was in motion, 
were probably in large measure loose, and rolled over each other as they 
were borne onward. The sand has found its way into all the interstices of 
these clinker-beds, and now binds the whole mass firmly together. At first 
sight, these bands might be taken for agglomerates of ejected blocks, and 
as already suggested, some of the slags may have been thrown out as loose 
pieces, but a little examination wdll show that in the main the rough 
scoriaceoiis lumps are pieces of the lava underneath. In these instances, 
(dso, it is clear that the blocks were in position before the fine sand was 
sifted into l.heir interspaces, for the pale green sandstone is horizontally 
Fig. 75. — Section sliowing the top of the volcanic scries at the 
foot of the precipice of tlie Re<l Head, Forfarshire. 
rr, Top of slangy andesite ; h, coarse volcanic coiigloinerafe ; c, He<l 
sandstone ; d, Tuff and volcanic conglomerate ; c, lied sandstones. 
