185 
1887.] Professor Geikie on Geology of St Alb’s Head. 
numerous, and this is probably the case with the amorphous ones 
also. Fluidal structure is occasionally marked, the microliths and 
lath-like crystals of plagioclase being grouped round the larger 
porphyritic ingredients. In some of the rocks minute amygdal- 
oidal cavities abound, and this even at a distance from the upper 
and under surfaces of the flows. The cavities are filled generally 
with calcite, or with calcite and quartz, or chalcedony. More rarely 
zeolites are present. The larger amygdules in the more scoriaceous 
portions of the rocks consist of the same minerals, calcite pre- 
dominating. Most of the rocks are more or less deeply stained 
with red ferritic matter. 
The tuffaceous areas in the porphyrites afford an interesting 
study. Occasionally they consist of broken or crushed scoriae of 
one and the same kind of rock completely embedded in porphyrite. 
These small lapilli or scoriae are generally finely and abundantly 
porous, and show a dark devitrified ground-mass in which minute 
microliths are more or less plentiful. They might quite well 
represent fragments of the more scoriacous and glassy portions of 
the same rock as that in which they are enclosed. In some places 
the tuffaceous matter consists of finely comminuted debris of the same 
or some closely similar rock, together with finer grit, and rounded 
pseudomorphs of serpentine and limonite after hornblende or 
pyroxene. The larger scorim and lapilli rarely exceed a hazel-nut 
in size. They are enclosed in the porphyrite in such a way as to 
show that this rock was in a fluid or pasty condition at the time 
they became embedded, for the crystalline and microlithic ground- 
mass lies between and among them. The tuffaceous areas now 
described appear to be confined to the upper and lower parts of the 
lava-flows, hut they occasionally occur nearer the middle. They 
are generally distinguished from the rock in which they are 
enclosed by their deeper red colour, a character which they have 
in common with the beautiful red tuffs of Horsecastle Bay. 
Bedded Tuffs . — These rocks present themselves amongst the por- 
phyrites at three horizons, hut it is quite possible that thin layers 
of similar fragmental materials, concealed by turf and superficial 
debris, may occur at other levels. A continuous section across the 
outcrops of the igneous series would probably show that each bed 
of porphyrite was underlaid by tuff or tuffaceous deposits. The 
