466 
THE CARBONIFEROUS VOLCANOES 
BOOK VI 
Over the greater part of the district, only fine tuffs were ejected. 
These occur as interstratifications among the ordinary sediments, and vary 
from mere thin partings, marking the feeblest and briefest explosions, up to 
continuous accumulations several hundred feet thick. As an example of the 
least vigorous emission of tuff I may refer to the sections already given on 
pp. 437, 438. The thicker bands are well illustrated by that which lies some 
way above the Houston Coal bet weenUrumcross and West Ifroadlaw in Linlith- 
gowshire, and by the great mass of tuff which occurs immediately below the 
Calmy Limestone on the Paver Avon near Linlitligow Bridge, and which 
may be 300 feet thick. ° 
_ It is a striking cliaracteristic of the tuffs that they may be met with in 
their solitary beds intercalated in the midst of ordinary sediments at a 
distance from any other trace of volcanic activity, their parent vents not 
being visible. I may cite in illustration an interesting case in the Swear 
Burn, near the southern end of tlie volcanic district. A band of tuff about 
ten feet thick lies there intercalated in a group of dark shales and thin coals. 
Below it there is a seam of coal four inches thick, and among the blue shales 
overlying it is another coal ten inches thick. The tuff is pale green, almost 
white in colour, hue in texBire, like a volcanic mud, while some of its com- 
ponent beds, one foot in thickness, are made up of fine laniinm and are even 
false -bedded. We might infer that the volcanic vent lay at some 
distance, so that only the finest dust fell over the swamps in which the coal- 
vegetation was accumulating, but for the presence of occasional lilocks of 
basalt one foot in diameter scattered through the tuff. When the eruptions 
ceased, the deposition of ordinary mud and the accumulation of plant-remains 
went on as before, and animal life crowded in again over the spot, for between 
the partings of the coal above the tuff, abundant fragments of eiirvpterids 
and scorpions may be found. 
One of the most extensive volcanic discharges of fragmentarv material 
was that which produced the “ Houston marls ” already referred to. These 
strata appear to mark a peculiar phase in the volcanic history of the Lower 
Carboniferous rocks of the Firth of Fortli, when exceedingly fine ash, or 
perhap.s^even volcanic mud, was erupted in considerable quantity. The 
“marls” attain in some places a thickness of nearly 200 feet, amr can be 
traced through most of the eastern part of Linlithgowshire, over an area of 
pel haps more than 50 square miles. This volcanic platform, which has been 
follovved in mining for oil-shale, is one of the most extensive among the puy- 
eruptions. The material, which probably came from one or more vents 
among the Batligate Hills, is not always of equal fineness, but passes into 
and even alternates with ordinary granular tuff. Thus in the Xiddry Burn, 
above Ecclesmachan, the dull sage-green and browni.sh red Houston marls 
contain a few inconstant layers of green tuff, in which may be noticed pieces 
of black shale and lapilli of the usual basic pumice. Not far to the west, 
beyond W ester Ochiltree, and thus probably nearer to the active vents that 
ejected the dust and ashes, the Houston marls are replaced by or include a 
bedded granular tuff or basalt-agglomerate, which lies above the 2-feet coal 
