1887.] Professor Geikie on Geology of St Abb’s Head. 183 
to fill up irregular clefts and vein-like cavites in tke porphyrite, 
while in other places it appears involved in the porphyrite in such 
a way as to suggest that it may probably consist of portions of the 
shattered scoriaceous crust of the porphyrite, broken up and incor- 
porated in the underlying mass while that was still in a fluid or 
pasty condition. Occasionally so much of this tuff-like matter is 
enclosed in the porphyrite that one is sometimes in doubt as to 
whether the whole rock is not fragmental. Microscopic examina- 
tion, however, clearly shows that this is not the case — the angular 
and sub-angular lapilli and cinder-like fragments being completely 
surrounded by or embedded in finely crystalline rock. The por- 
Fig. 4. — Red Tuff (t, t, t), enclosed in Porphyrite («, a, a). Enclosures vary 
from an inch or less up to a foot or more in diameter. 
phvrites are all more or less weathered and earthy to some depth, 
and it is thus difficult to obtain very fresh fractures. They form a 
series of low broken escarpments facing the north-west, each 
escarpment marking the outcrop of a bed. The beds appear to be 
of variable thickness, some measuring about 15 feet, while others 
mav reach as much as 50 feet or more. 
Examined under the microscope, these rocks show a ground-mass 
of colourless microliths and minute lath-like crystals of plagioclase, 
diffused through which there is usually more or less non-differenti- 
ated red ferritic matter. In some cases the ground-mass seems to 
be composed chiefly of this unindividualised matter, with micro- 
liths and small rods of plagioclase scattered less abundantly through 
it. This is more especially the case with the amygdaloidal parts of 
the rock, where occasionally the ground-mass consists almost ex- 
clusively of non-differentiated matter, only a few recognisable 
microliths making their appearance. There can be little doubt 
that this unindividualised substance is simply the result of devitri- 
