48 . The Irish Naturalist., 



brown, grey, and yellow sandstones, passing up into thinly 

 bedded micaceous, rippled sandstones, and sandy marls 

 referred to the lyower Keuper series of the midland counties of 

 England.' The strata near Cultra Point are traversed by trap 

 dykes, striking out to sea, which have greatly indurated the 

 sandstone. 



The Triassic rocks, overlain by Post-Tertiary gravels, ex- 

 tend round the curve of the bay till, about 200 3^ards south- 

 west of Cultra Pier, a fault brings up the lyower Carboniferous 

 shales.^ A little east of the pier interesting beds of magnesian 

 limestone are visible near to low water-mark. These were 

 first described by Dr. Bryce, F.G.S., in 1835,3 and from the 

 evidence of the fossils they contain, were referred both by 

 him* and by Professor Kings to the Permian system. 



All these strata are traversed by trap dykes, mostly small ; 

 a considerable one, however, occurs near Cultra Pier, where it 

 has greatly indurated the shale, and converted it into flinty 

 slate and hornstone rock.^ 



Here then, at the points where the strata are most hardened 

 and altered, the resistance has been the greatest, while along 

 the intervening curve of the bay where the cliff is softer, it has 

 fallen an easier prey to denuding forces. But the difference, 

 as we have seen, is only one of degree ; the whole area has 

 receded rapidly before the wasting influences to which it has 

 been exposed. 



The sandstones and shales could offer but feeble resistance 

 to the action of the sea, and small landslips have hastened the 

 destructive work. Winds and currents have aided, more 

 especially under certain conditions. A strong southerly wind 

 increases largely the rise of tide in Belfast lyough, and often 

 blows at night accompanied with rain. If the wind then veer 

 round to east or north-east a strong ground-swell sets in and 

 undermines portions of the cliff, already loosened by rain 

 and other sub-aerial agents. The material thus dislodged is 



1 Geol. Survey Memoir to sheet 29, p. 19. 



2 Ibid. 



^your. of the Geological Society of Dublin, vol. i., part 3. 

 * Report of the British Association for 1852. 

 5 Ibid. 



^ I have to thank Mr. S. A. Stewart, F.B.S.B. for his kindness in 

 examining specimens of the altered shale. 



