444 
THE CARBONIFEROUS VOLCANOES 
BOOK VI 
platform, a mass of ordinary sediment between the 
lavas or tnfls already erupted and those of suc- 
ceeding discharges, and thus to furnish valuable 
geological chronometers by wliich to define the 
stratigrapliical horizons of the successive phases of 
volcanic energy. 
The volcanic banks or ridges not improbably 
emerged as islets out of rhe water, and were some- 
times ten miles or more in length. Their materials 
were supplied from many separate vents along their 
surface, but probably never attained to anything 
approaching the elevation which they would have 
reached had they been poured out upon a stable 
platform. This feature in the history of the vol- 
canic ridges is admirably shown by the fact just 
referred to, that recognizable stratigrapliical horizons 
can sometimes be traced right through the heart 
of the thickest volcanic accumulations. One of 
the largest areas of basalts and tuffs connected 
with the puys is that of the Linlithgow and Bath- 
gate Hills, where, as already remarked, a depth of 
some 2000 feet of igneous rocks has been piled 
up. Yet several well-known seams of stone can 
be traced through it, such as the Hnrlet Lime- 
stone and the Inde.v Limestone (Fig. 155). Only 
at the north end, where the volcanic mass is thickest 
and the surface -exposures of rock are not con- 
tinuous, has it been impossible to subdivide the 
mass by mapping iutercalations of sedimentary 
strata across it. It would thus seem that, even 
where the amplest accumulations gathered round 
the puys, they formed low flat domes, rather than 
prominent hills, wdiich, as subsidence went on and 
the tuff-cones were washed down, gradually sank 
under water, and were buried under the accumulat- 
ing silt of the sea-floor. 
As a detailed illustration of the manner in 
which the growth of organically-formed limestones 
and the deposit of ordinary sediment took place 
concuirently with the occasional outflow of lava- 
streams over the sea-bottom, 1 may cite the section 
presented in another Linlithgowshire ipuirry (Fig- 
156). At the bottom of the group of strata there 
exposed, a pale amygdaloidal, somewhat altered 
basalt (A) marks the upper surface of one of the 
submarine lavas of the period. Directly over it 
