1887.] Professor Geikie on Geology of St Abb’s Head. 191 
a closing stage in the same period of volcanic activity. The dykes 
of Bell Hill and the harbour are certainly of much more recent 
date, and need not be considered in the present paper. 
The south-east dip of the bedded igneous rocks of St Abb’s I 
take to be due to the two large faults which form their boundary 
lines. The original inclination of the strata, which need not have 
been great, would necessarily be away from the neck of Coldingham 
Shore. It is quite impossible to say what the extent of these faults 
may be, but it is probably considerable, as the following considerations 
will show. Neither the top nor the bottom of the bedded rocks is 
seen, but the actual thickness displayed is not less than 1200 feet. 
At present the beds dip towards S.E. at about 15°. If, therefore, 
we take the beds at Pettico Wick to represent the basement beds of 
the igneous series, which they certainly are not, then, to restore the 
beds to horizontality, it is evident that the tuffs at the south-east end 
of the headland of St Abb’s would require to be lifted up for some 
1200 feet. But, on the supposition that all these bedded rocks have 
been derived from the volcanic neck at Coldingham Shore, and would 
at first, therefore, have a dip towards the north, it is obvious that the 
subsequent downthrow produced by the N.W. and S.E. fault must 
considerably exceed 1200 feet — say, 1300 or 1400 feet. A glance 
at the section seen at Pettico Wick, however, shows us that the beds 
there are not the basement beds of the series, for they are faulted 
down against the Silurian. The actual base of the series is submerged. 
If, therefore, the present S.E. dip of the porphyrites and tuffs be due 
mainly, as I believe, to the fault at present referred to, the down- 
throw of that dislocation must increase from N.W. to S.E., until it 
reaches not less than 1300 feet. To the effect produced by this fault 
we have to add that of the fault at Halterem’s Loup, which has a 
downthrow to N.W., and has therefore had its share in bringing 
about the S.E. dip of the bedded porphyrites and tuffs. It is im- 
possible, however, to form even an approximate estimate of the 
amount of downthrow produced by this latter fault. Erom the fact 
that it seems to cut off the other, no trace of which occurs in the 
rocks to the south, we may infer that its downthrow is considerable. 
Be that as it may, it is quite certain that the tuffs of Horsecastle 
Bay occupy a much higher horizon than the agglomerates exposed 
upon the beach at Coldingham Shore. 
