1887.] Professor Geikie on Geology of St Abb's Head. 
181 
strata can only be a fault, having its downthrow to the north-east. 
This is the dislocation already referred to as bringing down the 
nw SE 
S' Abbs Head 
PctticoWic/Z Lighthouse Ki rich ill White Heugh 
Fig. 2. — General Section across St Abb’s Head to Shore of Coldingham Bay 
(scale 3 in. to 1 mile ; horizontal and vertical scale the same). 
a, a, Probable position of Silurian strata under rocks of St Abb’s Head. 
b, Conjectural position of basement conglomerate of Old Bed Sandstone 
series, c, Inferred base of Volcanic series, p, p, p, Bedded porphyrites = 
andesitic lavas, t , t, t, Bedded porphyrite-tuff ; at ts largely composed of 
small scoriae. /3, (3, 13, Agglomerate and tuff in volcanic neck, ip, ip, ip, 
Intrusive porphyrite. f, f, Fault at Halterem’s Loup, a 1 , Silurian strata 
somewhat altered ; a 2 , Silurian strata much contorted and altered. 
bedded igneous rocks of the Old Red Sandstone series against the 
basement conglomerate of Bell Hill. It is well seen at Rutherford 
Brae. Another fault, running at right angles to that just described, 
forms the boundary between the Old Red and the Silurian on the 
south side of Bell Hill. It is seen in section in the sea-cliffs at 
Halterem’s Loup, where the Old Red Conglomerate is turned up at 
a high angle against the Silurian greywackes and shales (see 
fig. 3). 
Fig. 3.— Section across Bell Hill, showing relation of Basement Conglomerate 
to the Silurian strata. 
