CARBONIFEROUS VOLCANIC ROCKS OF THE FIRTH OF FORTH BASIN. 469 
tuff or agglomerate runs in veins and dykes, or fills up indentations in the 
boundary walls. This structure is illustrated by fig. 12, which represents a 
Fig. 12.—Ground-plan of Tuff-neck, shore east of Dunbar. 
(The surrounding rocks are sandstones, which are much hardened round the vent 
in the zone marked by the short diverged lines. See ‘Geology of East Lothian,” 
Mem. Geol. Survey.) 
ground-plan of a neck in the East Lothian district. In some cases, as already 
stated, particularly in the east of Fife, injections of a kind of volcanic mudstone 
have filled up rents in the surrounding rocks, so as to look like true lava dykes 
or veins. 
In the great majority of the shore-sections, a remarkable change of dip is 
observable among the strata round the edge of each vent. No matter what 
may be the normal dip of the locality, the beds are bent sharply down towards 
the wall of the neck, and are frequently placed on end. The structure 
(shown in figs. 6 and 8) is precisely the reverse of what might have been 
anticipated, and can hardly be due to the upward volcanic explosions. It is 
usually associated with considerable metamorphism in the disturbed strata. 
Shales are converted into porcellanite or various jaspery rocks, according to 
their composition. Sandstones pass into a distinct quartz-rock, with its 
characteristic lustrous fracture. It is common to find the vents surrounded by 
a ring of this altered sandstone, which from its hardness and vertical or highly 
inclined bedding, stands up prominently on the beach (as in fig. 12), and serves 
to mark the position of the necks from a distance. 
I have not been able to find an altogether satisfactory explanation of the 
inward dip of the strata round the vents. Taking it in connection with the 
metamorphism, I am inclined to believe that it took place after the long- 
continued volcanic action which had hardened the rocks round the volcanic 
pipe had ceased, and as the result of some kind of subsidence within the vent. 
VOL, XXIX. PART I. 6 E 
