Cou7ity Dublin, Past and Present. 33 



coral-reef and its associated shell-banks, is uptilted from 

 beneath more clearly stratified layers of alternating limestone 

 and shale, the latter, as seen in the picturesque little head- 

 lands (fig. 4), showing numerous folds and sharp contortions. 

 In the section by Mr. Du No3"er' the more shalj^ beds are repre- 

 sented as wholly above the limestone ; but detailed mapping 

 on a larger scale ma}^ prove that some of them are equivalent 

 to it, having been deposited upon the flanks of the main bank 

 of shells and coral at the same time as it was slowh-^ accumu- 

 lating. Such limestone aggregations very rapidly become 

 compact, and even crs'stalline in modern seas, and the waves 

 roll blocks of consolidated limestone from their margins into 

 the surrounding sedimeiits. At Portraine similar action has 

 produced bands of coarse conglomerate, which ma}' be seen 

 freely in the northern half of the coast section, and which are 

 folded in with the more finely grained deposits near the main 

 limestone mass. 



The fossils of the reef are partly silicified, owing, doubtless, 

 to the destruction of siliceous sponges, radiolarians, or diatoms, 

 and the deposition of their material by gradual replacement in 

 the calcareous shells and vskeletons of the other organisms. 

 This flint}^ character causes them to stand out above the level 

 of the rocks when attacked b}" waves and weather. Among 

 the corals, the isolated cups of Cyathophylliun, with numerous 

 septa, are conspicuous ; so is also the chain-coral, /2^a/;'^?7<?^, the 

 rows of tubes, in which the poh'pes once dwelt, resembling, 

 when viewed from above, the links of a chain. The brachiopods, 

 more closely than the actinozoa, ally these beds to the Bala 

 series of Wales. Orthis and StTophoinena have yielded several 

 species, as may be seen in Mr. Baih-'s useful list."" Thirteen 

 species of trilobites are on record, though their remains lie well 

 hidden ; and eight gastropods and two early representatives 

 of lamellibranchs have been found here or on Lambay Island. 

 In a band of black shale, running at right angles to the shore- 

 line, north of the more northern martello tower, the hydrozoan 

 Diplograptus can be found in occasional patches ; this grapto- 

 lite, and others with a similar arrangement of two rows of 

 cellules, back to back along the axis, are typically Ordovician ; 

 wdiile modern marine hydrozoa are common upon the shore, 

 ready for comparison. The sea- also throws up multitudes 

 of lamellibranchs, Cyprina, Cardiitm, Solen, and the like, and 

 gastropods, such as Ttinitella^Ptiipjwa, Buccimnn, and Cypr<Ta\ 

 but not a single brachiopod, nor a single coral is likely to 

 reward the most careful search. These animals are still 

 represented in British and Irish seas, but the proportions of 

 the several groups of marine life have been utterly reversed 

 since Ordovician times. Moreover, scarcely a single genus. 



* Memoir of Geol. Survej- of Ireland to sheets 102 and 112, p. 45. 

 ■2 Memoir of Geol. Survey to sheets 102 and 112, p. 12. 



