4i6 
THE CARBONIFEROUS VOLCANOES 
BOOK VI 
continuous floor of calcareous material over the whole area of central Scot- 
land. In these circumstances, the puy-eruptious of that basin would be 
long subsequent to the eruptions of the Clyde plateau, as they certainly 
were to those of the plateaux of Midlothian and the Garleton Hills. 
In tracing tlie geographical distribution of the puy -eruptions we are first 
impressed with the force of the evidence for their exti'emely local and 
restricted character (Map IV.). Thus in the area of the basin of the Firth 
of Forth, which may be regarded as the typical region in Britain for the study 
of this form of Carboniferous volcano, traces of them are abundant to tlie 
west of the line of the Pentknd Hills. To the east of that line, however, not 
a vestige of puy-eruptions, save a few sills of uncertain relationhip, cak be 
detected, though the same series of stratigraphical horizons is well developed 
on both sides of tlie Lothian coal-field. Again, to the westward of the 
Forth basin over the area of Stirlingshire, Lanarkshire and Eenfrewshire 
lying to tlie north of the great volcanic plateau, no record of puy-eruptions 
lias been noticed. Immediately to tlie south of that plateau, however, these 
eruptions were numerous in the north of Ayrshire. Yet the rest of the 
Carlxmilerous area in that large county has supplied no relics of these 
eruptions save at one locality — tlie Heads of Ayr. Lastly, while no trace 
of any younger display of volcanic activity occurs in the Merse of Berwick- 
shire, east of the plateau series of that district, the ground immediately to 
the west abounds in puys, and contains likewise extensive sheets of tuff and 
beds of basic lavas connected with these vents. 
Another fact which at once attracts notice in Scotland is the way in 
which the puy-vents have generally avoided the areas of the plateaux, 
though they sometimes approach them closely. As a rule, it is possible 
to tlistinguish the tuffs and agglomerates which have filled up these vents 
from those that mark the sites of the eruptive orifices of the plateaux. 
There are, no doubt, some instances, as in Liddesdale, where puys have 
appealed on the sites of the older lavas, but these are exceptional colloca- 
tions.' On the other hand, many examples may be found where puys have 
risen in^ the interspace between the limits of the eruptions of two plateau- 
areas. Thus the tract between the Clyde plateau-eruptions on the west 
and ^ those of the Garleton Hills on the east was dotted over with puys. 
Again, the southern margin of the Clyde plateau in Ayrshire, from Dairy to 
Galston is flanked with puys and long sheets of their lavas and tuffs.^ 
ii. NATURE OF THE MATERIALS ERUPTED 
A. The Lava-form Eocks 
We have now to consider the nature of the materials erupted by the 
volcanic activity of the puys. The geologist who passes from the study 
1 A iiiean.'i of definitely placing some of these vents in the series of puy-eruptions is stated 
further on, at p. 476. 
- Reference may again be made here to the remarkable similarity between the Scottish Car- 
boniferous puy-vents and those of older Tertiary time in the Swabian Alps so fully described by 
