190 Proceedings of Royal Society of Edinburgh, [afril 4, 
numerously in a dull ferritic base. Patches and irregular aggregates 
of ferric oxide are common enough also, and probably represent horn- 
blende or pyroxene. The less altered specimens often show a devitri- 
fied base, crowded with microliths and small crystals of plagioclase. 
Scattered through this ground-mass are many larger crystals of 
plagioclase. Upon the whole, highly porphyritic rocks appear to 
be of more frequent occurrence in this agglomerate than they are in 
the bedded series of porphyrites and tuffs at St Abb’s Head. 
The unstratified agglomerate is irregularly traversed by masses 
of close-grained porphyrite, which has the same general character 
as the intrusive porphyrite near St Abb’s Head. This porphyrite 
is, for the most part, much reddened with iron oxides, and con- 
siderably weathered, so that fresh fractures are not readily obtained. 
Where least weathered, it is a somewhat compact blue or purplish 
rock. Here and there plagioclase felspar is conspicuous as a por- 
phyritic ingredient, and now and again pyroxene (augite or diallage) 
or pseudomorphs after pyroxene are visible. Thin veins and threads 
of limonite and haematite are common.* 
IY. General Conclusions as to the Old Red Sandstone Series. 
The basement beds of this series, consisting of the conglomerate 
of Bell Hill, are a mere fragment, and we cannot say much about 
the conditions under which they were accumulated. They show us, 
what indeed may be learned in many other parts of Scotland, that 
the Lower Silurian strata had already been folded, crumpled, and 
contorted, and excessively denuded before Old Red Sandstone 
times. As the conglomerates contain not a single fragment of 
igneous rock, it is most probable that their formation preceded 
the outbreak of volcanic action in this neighbourhood. 
The coarse tuff and agglomerate of Coldingham Shore fill up an 
old volcanic orifice, from which the porphyrites and porphyrite- 
tuffs of St Abb’s had previously been ejected. The great intrusions 
of porphyrite which penetrate, not only the agglomerate of the 
“ neck,” but the bedded tuffs and porphyrites, evidently belong to 
* A dyke of basalt-rock crosses the intrusive porphyrites at the harbour, 
Coldingham Shore. Unfortunately, I neglected to bring away a specimen of 
the rock for closer examination. But I hope soon to revisit the district 
for further study, and to supply the omission in a subsequent paper to the 
Society. 
