CHAP, XXVII 
VENTS OF THE SCOTTISH PUYS 
429 
derived from previous tuffs which, interstratitied iuiioiig the sedimentary 
strata, had been broken up by the opening of a new vent. But probabh' 
in most cases they should be regarded as portions of the volcanic debris 
whieli, having solidified inside the crater', was blown out in fragments by 
subsequent explosions. In a modern volcano a considerable amount of 
stratified tuif may be formed inside the crater. The ashes and stones 
thrown out during a period of activity fall not only on the outer slopes of 
the cone, but on the steep inner declivities of the crater, where they arrange 
themselves in beds that dip at high angles towards the crater bottom. This 
feature is well seen in some of the extinct cones in the Neapolitan 
district. In some of the Scottish puys the tuff is stratified and has tumbled 
down into a highly inclined or vertical position (Fig. 145). 
As a good illustration of the variety and relative proportions of the 
ejected blocks in the green tuff of the puy-vents, I may cite the following 
table of ])erceutages which I took many years ago in the tuff which rises 
through the Cement-stone group on the beach at the Heads of Ayr. 
Diabase and basalt . . . . 
Older tuff ..... 
Andesite (probably from Old Red Sandstone volcanic 
series of the neiglibourliood . 
Limestone (cement-stone, etc.) 
Shale ...... 
Sandstone . . . . . 
Fossil wood . . . . . 
57 
3 
per cent. 
5? 
14 
■20 
3 
2 
1 
100 
While many examples might be cited where no molten rock of any 
kind has risen in the vents, or where at least all the visible materials are of 
a fragmentary character, yet small veins and dykes of basalt have not 
infrequently been injected into the tuff or agglomerate. These seldom run 
far, and usually present a more or less toi'tuous course. Thus, on the south 
front of the Binn of Burntisland (Figs. 166, 168) a number of basalt-dykes, 
which vary in breadth from five or six feet to scarcely so many inches, 
bifurcate and rapidly disappear in the tuff, one of them ascending tortuously 
to near the top of the cliff. They at once recall the apjrearance of the well- 
known dykes in the great crater wall of Sonima. 
Though not by any means the largest or most perfect of the vents 
in the basin of the Firth of Forth, the Binn of Burntisland, of which a 
view is given in Fig. 144, may be cited in illustration of their general 
characters. It presents in detail some of the most strikingly volcanic 
aspects of scenery anywhere to be seen in that region. Consisting 
of a dull green granular volcanic tuff, it rises abruptly out of the Lower 
Carboniferous formations to a height of 631 feet above the sea. The 
southern edge of this neck has been so extensively denuded, that it 
presents steep craggy slopes and rugged precipices, which descend from 
the very summit of the cone to the plain below — a vertical distance of 
