1887.] Professor Geikie on Geology of St Abb’s Head. 187 
porphyrites — the stones being set in a meagre matrix of comminuted 
debris. These rocks dip S.E. at 18° to 20°. They are abundantly 
pierced and traversed by dykes and irregular sheets and masses 
of porphyrite, which will be described later on. In some places, 
especially in the area extending from Raven’s Brae to Horse Castle, 
the tuffs have been considerably baked and altered, so that it is 
sometimes difficult in hand-specimens to distinguish between what 
is tuff and what intrusive porphyrite. The fragmental character of 
the altered tuff, however, is quite apparent under the microscope. 
The fragments in these tuffs appear to consist exclusively of 
varieties of porphyrite ; at all events I could find no other rock. 
Most of the lapilli are vesicular and amygdaloidal — very many are 
highly so, and have all the appearance of scoriae. The amygdules 
being usually white are generally very conspicuous. A microscopic 
examination of many of these included fragments shows that?, they 
consist exclusively of porphyrite— the ground-mass being generally 
vitreous, stony, or devitrified, but occasionally microlithic. The 
description given above of the small lapilli, which occur in the 
tuffaceous areas of the bedded porphyrites, holds equally true of the 
lapilli of these bedded tuffs. A great number of the small stones 
are evidently fragments of a vitreous scoriaceous porphyrite, and from 
their highly vesicular character they might well have floated in 
water at the time of their ejection — they are, in short, mere cinders. 
The fragments which show a microlithic ground-mass often contain 
also pseudomorphs after hornblende or pyroxene, and are evidently 
true porphyrites. Red ferritic matter saturates most of the stones ; 
but not a few of these are dark grey or blue, and it is such 
fragments which show best the original vitreous character of the 
ground-mass. Here and there amongst the fine comminuted matter, 
or dust and grit, of the tuffs, small patches of serpentine and 
limonite occur, and these, in some cases at least, alniost certainly 
represent hornblende or pyroxene. 
The tuffs are thus composed essentially of volcanic detritus. In 
none of the specimens examined by me did I meet with any trace of 
ordinary terrigenous sediment. Even the finest-grained portions 
appear under the microscope to consist of minute fragments of 
porous scoriaceous vitreous rock. There is no admixture of quartz- 
grains and argillaceous matter, such as might have been derived from 
