192 
Proceedings of Boyal Society of Edinburgh, [april 4, 
While the geological structure of St Abb’s Head thus leads to 
the conclusion that the bedded tuffs and porphyrites formerly 
dipped in a northerly direction, and may thus have been ejected 
from the volcanic focus in their neighbourhood, the petrological 
evidence lends additional support to this conclusion. Fragments 
of precisely the same porphyrites as those of St Abb’s Head occur 
abundantly in the rock at Coldingham Shore. A few blocks and 
fragments of greywacke were observed in the agglomerate, and 
may, perhaps, be more plentiful than they seem to be, for, of 
course, the agglomerate is only partially exposed ; but with these 
exceptions all the stones I saw were porphyrites. Sections of the 
finer-grained portions of the tuff examined under the microscope 
showed in like manner that these are composed of the comminuted 
debris of porphyrites. 
From the fact that the bedded porphyrite-tuffs of St Abb’s Head 
have evidently been arranged by and accumulated under water, we 
may infer that the whole series, so far as that is exposed, is of 
subaqueous origin — that, in the rocks now described, we have the 
relics of an old subaqueous volcano of Old Red Sandstone times. The 
bedded porphyrites offer many analogies with those of the Cheviot 
Hills, the Sidlaws, and other regions of Old Red Sandstone volcanic 
rocks. They are, in fact, only altered andesites, which, in their micro- 
scopic structure, reproduce exactly what one sees in the andesites of 
the Western Territories of North America. The intrusive porphy- 
rites, however, which intersect the agglomerate and the bedded rocks 
hardly resemble the intrusive masses of the Sidlaws, the Cheviots, 
&c., those of the Sidlaws being mainly diabase (altered basalt-rock), 
while those of the Cheviots are chiefly granite and felsite. Again, 
the bedded scoriee-tuffs of St Abb’s Head are hardly paralleled by 
any tuffs met with either in the Cheviots, the Sidlaws, or any other 
region of Old Red Sandstone volcanic rocks known to me. Of 
course, lapilli of highly porous and amygdaloidal porphyrites occur 
commonly enough in many of those districts, but nowhere, so far 
as I have seen, have we such a depth of fragmental materials con- 
sisting almost exclusively of scoriae or cinders. These form the 
highest beds of the series, and possibly represent the latest ejections 
from the adjoining volcanic orifice. But they have suffered great de- 
nudation, and certainly attained a greater thickness, and overspread 
