CHAP. XVIII 
arrangement OF BEDDED LAVAS AND TUFTS 
285 
The part taken by the Tuffs in the structure of the ground agrees with 
what might have been expected in the accompaniments of extremely slaggy 
and viscid lavas. These pyroclastic intercaktions are, m most of the 
volcanic districts, comparatively insignificant m amount, by far the largest 
nronortion of solid material ejected from the various vents having consisted 
of streams of lava. Bound or within some of the vents the fragmentary 
materials attain a remarkable coarseness, as may be seen m the great 
a-glomerates of Dumyat, near Stirling, the largest of which is more than 
TOO feet thick. These massive accumulations doubtless represent a long 
series of explosive discharges from the summit of the lava column in one or 
more adjacent vents. Traced away from the orifices of emission the tuffs 
rapidly grow finer in grain, less in thickness, and more mixed with ordinary 
detritus, until they pass into ordinary iion-volcanic sediment or die out 
between the lava-sheets. .1 ^.i • • 
Good sections, showing the nature and arrangement of the thin inter- 
calations of andesite-tuff between the successive outpourings of lava, may be 
examined on the coast. Thus, near Turnherry Point, in Ayrshire, upwards 
of a dozen successive flows of lava, with their sandy and ashy intervening 
layers, are exposed in plan upon the beach, and partly also in section along 
the cliffs on which the ruins of the historic castle of Turnherry stand. 
(Figs. 95, 96, 97). Again, along the coast of Forfarshire, from the Be 
Head to Montrose, the numerous sheets of andesite are separated by layers 
of dull purplish tuff passing into conglomerate, with blocks of porphyrite a 
yard or more in diameter. /-m 1 1 c 
The most remarkalile interstratified tuffs in the Lower Old Bed Sand- 
stone are the felsitic varieties. Those which proceed from tlie great vent of 
the Braid Hills, extend south-westwards for eight or nine miles, and then 
peculiar materials, mixed with ordinary sediment, may be traced several miles 
further. They occur in successive sheets, which, from a maximum thic '- 
iiess and number at the north end, gradually thin away southwards, like 
the felsitic lavas which they accompany, and from the explosion of whmli 
they no doubt were derived. They consist to a large extent of extremely fine 
volcanic dust, and since they are generally much decomposed, it is often as 
already remarked, hardly possible to distinguish between them and he 
equally decayed felsites. In some parts ,of the hills they present a dist 
fiLile ^bedding; but still more satisfactory is the occasional fine brecciated 
structure which they assmne, when they are seen to consist of angulai 
lanilli of different felsites. • * 
The amount of volcanic material ejected from the more important vents 
was much greater than the height of the present hills would lead us to 
suppose The rocks have generally been tilted into positions much more 
indined than those which they originally occupied, so that to measure then 
actual thickness we must take a line approximately perpendicu ar to the 
dip. In this way we ascertain that the accumulated mass^ of lavas and 
tuffs immediately outside the vent at the north end of the Pentland Hills 
must be at least 7000 feet thick, for the base of the series is concealed 
