180 Proceedings of Royal Society of Edinburgh, [april 4 , 
be exclusively intrusive in character. I shall describe the rocks of 
those two areas separately, although they probably all belong to the 
same period of volcanic activity. 
5W Bell Hill NE 
Fig. 1 . — Section across Bell Hill, showing relation of Basement Conglomerate 
(c) to Igneous Rocks of St Abb’s Head (t, ip). 
(a) Rocks of St Abb's Head. 
The headland of St Abb’s Head extends from Pettico Wick south- 
east to the W r hite Heugh — a picturesque cliff and favourite resort of 
sea-birds — about J of a mile north-west of Coldingham Shore. The 
headland presents to the sea a bulwark of wild rugged precipices, in- 
dented with numerous little bays and coves, only a few of which are 
accessible from the land. It is separated from the rolling Silurian 
uplands behind by a well-marked hollow that extends south-east from 
Pettico Wick in the direction of the White Heugh. The headland, 
as thus defined, is described by the Geological Survey as consisting 
in the south-east partly of fragmental igneous rocks, and partly of 
intrusive “felstone”; while between Horsecastle Pay and Pettico 
Wick the area is coloured as intrusive felstone alone. Various sections 
laid bare since the time the ground was examined by the Geological 
Survey show that the northern part of the headland is made up 
chiefly of bedded porpliyrite with some intercalated layers of tuff. 
Of the latter the only good exposures seen are at Pettico Wick 
Harbour and on the side of the road leading thence to the light- 
house. The dip of these rocks is clearly towards the south-east, 
the whole forming an ascending series, from Pettico Wick to the 
Wuddy Eocks, with an approximate thickness of 1200 feet (see 
fig. 2). Towards the south-east the beds are invaded by larger and 
smaller masses and dykes of intrusive crystalline rock. As the dip 
of the igneous series does not exceed 18° on an average, it is obvious 
that the junction between these rocks and the vertical Silurian 
