186 Proceedings of Royal Society of Edinburgh, [april 4 , 
lowest bed of tuff is seen at Pettico Wick Harbour. ‘ This is a coarse 
tuff or agglomerate of fragments of porphyrites, which are generally 
dark purplish blue and red in colour, and more or less highly 
amygdaloidal. The stones are angular and subangular in form, and 
show no regular arrangement. They vary in size from small lapilli 
up to blocks of more than 1 foot across — some measuring 2 feet in 
diameter. Examined under the microscope, these porphyrites show 
the same structure and composition as those already described. The 
tuff rests upon a very irregular surface of porphyrite, and is overlaid, 
in a like irregular manner, by a bedded porphyrite, which is very 
vesicular and amygdaloidal at the line of junction. The junction is 
somewhat confused in places by minor slips and faults. 
On the side of the road leading from Pettico Wick to the light- 
house various porphyrites are exposed, — some of which show the 
curious vein-like and irregular inclusions of fine tuff already de- 
scribed. These beds are overlaid by a considerable thickness of 
well-bedded shaley tuffs, generally red in colour — the tuff being fine- 
grained, and composed of comminuted debris of porphyrites. Some 
of the beds contain many lapilli of larger size scattered through their 
mass. They rest upon a dark purplish-blue amygdaloidal porphyrite, 
and are traversed intrusively by a thin sheet of fine-grained porphyrite. 
The junction is slightly confused, as at Pettico Wick, by faulting. 
But by far the most extensive succession of tuffaceous strata is 
that which occurs at the south-east end of the headland of St Abb’s. 
These rocks are generally well bedded, and have a prevalent red 
colour. They vary in texture from tuffaceous mudstones, in which 
only grit and small lapilli occur, up to coarse-grained tuffs, in 
which the included fragments may reach 1 foot or more in diameter. 
The most common rock is a tuff composed of comminuted debris 
and small lapilli, often not larger than a hazel-nut in size, but some- 
times measuring 2 or 3 inches across. The grit and lapilli are of all 
shades of red, purple, yellow, and blue — the red strongly predomi- 
nating — so that the resulting tuff is finely mottled. In many cases 
decomposition products, particularly calcite, permeate the rock in 
all directions — giving rise to irregular white splatches, veinings, and 
tangled thread-like areas — which show well on the warm red ground 
of the tuff. Other parts of the tuff might be described as a breccia 
of subangular and angular fragments, chiefly of amygdaloidal 
