CARBONIFEROUS VOLCANIC ROCKS OF THE FIRTH OF FORTH BASIN. 461 
It is to be observed that the tuff in the necks of each district partakes of 
the nature of the lava emitted in that district. In the East Lothian and 
Stirlingshire areas, for example, where the lavas were the so-called porphyrites, 
the tuff consists of the debris of these rocks. Elsewhere among basaltic lavas, 
the tuffs have a characteristic dirty green colour, and in this as well as in other 
respects show that they have been derived from these rocks. The ejected 
fragments contained in the tuffs bear the same relation to the surrounding 
lavas. In those cases where, as in so many of the vents of the east of Fife, no 
lava flowed out at the surface, we can yet tell from the character of the 
abundant ejected fragments what was the nature of the molten rock which 
ascended the volcanic chimney, and produced by its ebullition the abundant 
showers of tuff. 
The lava blocks in the tuffs and agglomerates are usually rounded or sub- 
angular. Pear-shaped blocks or flattened discs or hollow spherical balls are 
hardly ever to be observed, though I have noticed a few examples in the 
tuffs of Dunbar and Elie. A frequent character of the blocks is that of © 
roughly rounded, highly amygdaloidal pieces of lava, the cellular structure 
being specially developed in the interior, and the cells on the outside being 
often much drawn out round the circumference of the mass. Blocks of this 
kind, two or three feet in diameter, may be seen at some of the Elie vents. 
They were probably torn from the cavernous, partially consolidated, or at least 
rather viscous, top of a lava column. Most of the stones, however, suggest 
that they were produced by the explosion of already crusted lava, and were 
somewhat rounded by attrition in their ascent and descent. The vents filled 
with such materials must have been the scene of prolonged and intermittent 
activity ; successive paroxysms resulting in the clearing out of the hardened 
lava column in the throat of the volcano, and in the rise of fresh lava, with 
abundant ejection of dust and lapilli. Corroborative evidence that the 
intervals of explosion were separated by long periods of quiescence is fur- 
_hished by the fragments of wood to be afterwards referred to, and likewise by 
the numerous pieces of stratified tuff frequently to be noticed imbedded with 
the other debris in a neck. These angular blocks of older tuff resemble in 
‘general petrographical character parts of the tuff among which they are 
imbedded. There can be little doubt that they are portions of the volcanic 
debris which solidified inside the crater, and which was blown out in fragments 
by subsequent explosions. In a modern volcano a considerable amount of 
Stratified tuff 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 which dip at high angles towards the crater bottom. This feature is 
well seen in some of the extinct cones in the Neapolitan district. At Astroni, 
VOL, XXIX. PART I. 6c 
