CHAP, xvni 
BEDDED LAVAS AND TUFFS— VENTS 
387 
together as shingle-beaches, and were derived in part from volcanic cones 
which liad risen above the level of the lake. Tliey seem to suggest con- 
siderable degradation of these cones by breaker-action, whereby blocks ol 
rock a yard or more in diameter could be rounded and smoothed. 
Another inference deducible from such conglomerates, and to which I 
have alreiidy alluded, is that considerable intervals of time took place 
between some of tlie eruptions. Round the vents, indeed, where the 
successive sheets of volcanic material follow each other continuously, it is 
perhaps impossible to form any definite opinion as to the relative chiono- 
logical value of the lines of separation between different ejections. But 
where some hundreds of feet of coarse conglomerate, chiefly composed of 
well-rounded andesite blocks, intervene between two streams of lava, we 
may conclude that tlie interval between the outpouring of these .streams 
must have been of considerable duration. Other evidence of a similar 
tendency may be recognized in the intercalation of groups of varied sedi- 
mentary accumulations, such as those which were deposited over the site of 
Eastern Forfarshire and Kincardineshire during the time tliat ela])sed 
between two successive floods of lava. In the Den of Oanterland, tor 
example, in the midst of the volcanic sheets we find interesting evidence of 
one of these intervals of (piiescence, during which layers of fine olive shales 
were laid quietly down, while macerated vegetation, drifting over the lake- 
bottom, was buried with remains of fishes, and abundant gally-worms 
{Kani'pecaris, Arclddesimis), washed from the neighliouring land.^ So un- 
disturbed were the conditions of deposition that calctireous sediment gatliered 
round some of tlie organisms and encased them in limestone nodules. 
In some of the districts the discharges of volcanic material were so 
abundant or so continuous that no recognizable deposition of ordinary 
sediment has taken place between them. Thus, at the north end of the 
Rentland Hills the rocks are entirely of volcanic origin, and though, as we 
trace them southwards away from tlie centre of eruption, they diminish in 
thickness, they include hardly any interstratified sandstones and conglomer- 
ates until they finally liegin to die out. 
The distances to which the lavas and tuffs have been erupted from the 
chief vents of a district vary up to 15 or 20 miles. Those of tlie Rentland 
Hills extend from the Braid Hill vent for 10 miles to the south-west. 
Those of the Biggar centre stretch for about 16 miles to the north-east. 
Those of the Ochil Hills, which probably came from a number of distinct 
vents, can be traced for nearly 50 miles. 
ii. VENTS 
On the whole the actual vents of the volcanoes of Lower Old Red Sand- 
stone time are leas clearly distinguishable than those of subsequent volcanic 
1 An abundant organism in some of these dejiosits, named Farka, was first regarded as a plant, 
was afterwards believed to be the egg-paekets of erustaoea, and is now pronounced by competent 
authorities to belong to an aipuatic plant with creeping stems, linear leaves and sessile sporo- 
carps. 
