1887.] Professor Geikie on Geology of St Abb's Head. 179 
remarkable feature presented by these felsites is the appearance of 
minute veins which traverse the felsite irregularly, not infrequently 
crossing and anastomosing with each other at all angles. They 
vary in width from a mere line up to t L- or i of an inch in dia- 
meter, and consist chiefly of quartz and felspar, apparently ortho- 
clase. The quartz undoubtedly predominates, but here and there 
felspar, or saussurite which replaces the felspar, forms the chief 
ingredient. The quartz occurs in irregular crystalline aggregates 
and granules, often crowded with fluid cavities, and frequently 
containing enclosures of saussurite, and occasionally epidote. The 
felspar also forms irregular crystalline aggregates, but is most 
usually replaced by saussurite. Not infrequently it forms inter- 
growths with the quartz, so as to give the veins a micropegmatitic 
structure. Sometimes the walls of the veins are smooth and even ; 
at other times the quartz and felspar (saussurite) seem to indent 
the walls, as in the so-called “ segregation veins ” of granite. The 
veins now described appear to be confined, as a rule, to the felsite, 
but occasionally they pass outwards from the latter into the 
adjacent greywacke. 
III. The Old Red Sandstone Series. 
The rocks assigned to the Old Red Sandstone are principally of 
igneous or aqueo-igneous origin. There is one small patch of 
conglomerate, however, which forms the upper portion of Bell Hill, 
near the village of Coldingham Shore. This is indicated on the 
Geological Survey’s map as of Upper Old Red Sandstone age. I 
think it really belongs to the lower division of the series ; at all 
events it is older than the bedded igneous rocks to be described 
presently. It rests directly upon the Lower Silurian, and is in fact 
composed exclusively of rounded fragments of greywacke, &c., 
derived from that formation. It nowhere overlies the igneous 
rocks referred to, nor does it contain any admixture of fragments of 
these. Its junction with them is, in short, a dislocation or fault, 
which has a downthrow to the N.E. (see fig. 1). 
The bold headland of St Abb’s is composed entirely of crystalline 
and fragmental igneous rocks, some of these being bedded and 
contemporaneous, and others amorphous and intrusive, while the 
igneous rocks at Coldingham Shore and Coldingham Bay appear to 
