1887 .] Professor Geikie on Geology of St Abb's Head. 189 
two places, near their junction with the latter, patches of tuff and 
agglomerate occur upon the foreshore, as if resting directly upon the 
Silurian ; but these patches are evidently of an intrusive character, 
and appear to occupy vertical fisures in the surrounding greywackes, 
which are extremely hardened and altered and very much reddened. 
The actual junction of these greywackes with the main mass of the 
igneous rocks presently to be described is likewise strongly suggestive 
of the intrusive character of the latter. Similar appearances mark 
the junction between these rocks and the Silurian greywackes on 
the shores of Coldingham Bay. At that place the greywackes are 
much jumbled, broken, and hardened, and saturated with red ferritic 
matter. The Old Red igneous rocks do not overlie, but are intrusive 
in the Silurian strata. 
These igneous rocks consist partly of tuffs and agglomerates and 
partly of porphyrite. The fragmental rocks are, for the most part, 
quite unstratified, although here and there some trace of rude bedding 
may be noted. They are of a dull red colour, and are made up of 
angular and subangular fragments of all shapes and sizes up to 
blocks measuring over a yard in diameter. These are set in a matrix 
of smaller stones and comminuted debris. The whole appearance 
of this coarse tuff, which in many places is quite an agglomerate — 
that is to say, a coarse breccia of large stones and blocks — is similar to 
that of those tuff's and agglomerates which elsewhere in Scotland are 
found occupying the necks or throats of old volcanic orifices. All 
the fragments consist of varieties of porphyrite — a great many of 
which are precisely similar to the bedded porphyrites of St Abb’s 
Head; others, however, are more markedly crystalline and porphyritic, 
especially with plagioclase felspar. A number of these fragments 
were examined under the microscope, but no new features of 
importance were disclosed. Highly amygdaloidal and scoriaceous 
fragments are common enough, but are not so strikingly prevalent 
as in the well-bedded tuffs of Horsecastle Bay, &c. One may say 
that, while in these latter scoriae predominate and fragments of less 
porous rock do not abound, in the agglomerates of Coldingham Shore 
it is just the reverse. From the lapilli and blocks of this agglomerate, 
however, I did not succeed in getting any fresh specimens. Most of 
the rocks are much altered, so much so indeed that all one can deter- 
mine is the fact that clouded crystals of plagioclase occur more or less 
