436 
THE CARBONIFEROUS VOLCANOES 
BOOK VI 
axis oil the opposite side of the Firtli. But if we restore the rocks to a 
horizontal, or approximately horizontal position, we find the Binn of Burnt- 
island rising among them in one or more necks, which doubtless mark 
centres of volcanic activity in that district. A series of smaller neck-like 
eminences runs for two miles westward. 
Striking as the forms of many of the necks 
are, and much as their present conical forms resemble 
those of active and extinct volcanoes, the evidence 
of extensive denudation proves that these contours 
are nut the original outlines of the Carboniferous 
vents, but are in every case the result of prolonged 
waste. What we now see is a section of the vol- 
canic chimney, and the conical form is due to the 
way in which the materials filling the chimney have 
yielded to the forces of denudation. 
Vi. ii. BEDDED TUFFS AKD LAVAS 
B -J 
I During at least the earlier part of the period of 
•= 1 1 the puys, in some districts or from certain vents, 
I I”, such as those of East Fife, Western Midlothian, 
I f-’S E^^tern Linlithgowshire, Northern Ayrshire, Heads 
^ .= 1 of Ayr and Lower Eskdale, only fine tuff seems to 
0 £ have been thrown out, which we now find inter- 
1 calated among the surrounding strata. These erup- 
2 l| neither so vigorous nor so long-continued as 
I "I those of the plateaux, never gave forth such thick 
I Tm and widespread sheets of fragmentary materials a.s- 
I I those associated with the plateaux in East Lothian 
■g § and the north-east of Ayrshire. A single discharge' 
“I of ashes seems in many cases to have been the sole 
S' I achievement of one of those little volcanoes ; at least 
d J only one thin band of tuff may be discoverable to 
^ mark its activitjL 
1 The tuff of these solitary bands is seldom coarse- 
I in texture. It usually consists of the ordinary dull 
I green 2 >aste, with dust and lapilli of basic pumice. 
The local vai’iations in the tuffs of the puys generally 
arise mainly from diilerenees in the composition, size 
and numbers of the included ejected blocks. Gener- 
ally the most abundant stones are pieces of different diabases, or basalts ; 
then come fragments from the surrounding Carboniferous strata, from older 
tuffs and rarely from rocks of much deeper-seated origin. 
Now and then the eruptions of tuff have consisted of extremely fine 
volcanic dust, which, mingling 'with water, took the form of a compact mud- 
stone, as in the case of the Houston Marls (p. 423), which remind one- 
