74 The Irish Naturalist. 



But, as Mr. G. H. Kinahan' strongly urges, such conglome- 

 rates recur at various levels in the Carboniferous. No large 

 mass of land can be uniformly and suddenly submerged. The 

 shore-line, during subsidence, graduall}^ recedes ; islands are 

 left for a time, finally to be buried in the sediments ; and froui 

 their flanks torrents occasionall}^ roll down coarse material, 

 forming irregular bands of conglomerate among the more 

 normal deposits. Thus on the projecting rocks about one 

 and a half miles south of Skerries, a bed occurs in the Carbo- 

 niferous full of partly rounded flakes of Ordovician shale and 

 pebbles of yet older quartz. The Ordovician is exposed at 

 Skerries itself, and at Shenick's Island immediately opposite ; 

 and thus at the present day marine beaches are forming in 

 this area in precise repetition of those of Carboniferous times. 

 More striking conglomerates can be seen at the southern side 

 of the entr>" to Rush harbour, forming considerable beds in 

 the steeply dipping Carboniferous series. The coarseness of 

 the materials shows that land was not far off at the time of 

 their deposition. 



In its more finel}^ grained deposits the Carboniferous system 

 of Co. Dublin also bears evidence of abundant foreign material. 

 Much of the limestone, formed in the deepening sea by the 

 accumulation of shells, crinoid-stems, etc., assumes a black 

 argillaceous character; such beds have been st3ded the "Calp." 

 If the rock is dissolved in acid, the black clay is separated, 

 and forms a fine mud in the bottom of the vessel. As has 

 often been pointed out," this mud was derived from the neigh- 

 bouring .spurs and islands formed of Ordovician or older rocks. 



In the south of the county, the foreign bodies in the "calpy" 

 limestone give us conclusive proof as to the antiquity of the 

 granite chain. Both schistose fragments from the altered 

 Ordovicians, as ma}^ be seen in the stones used for the 

 Booterstown sea-wall, and granite fragments, as in the quar- 

 ries of Milltown and Crumlin,^ are embedded in the limestone, 

 and prove that the igneous mass had consolidated before the 

 Carboniferous sea invaded the countr}^ Dr. Haughton has 

 observ^ed pieces of granite in the limestone as much as eight 

 inches in diameter. The sea clearl}^ wrapped round the flanks 

 of the lycinster chain, and continued subsidence finally allowed 

 of the deposition of Carboniferous strata sufficient to bury 

 even the highest summits. 



The great mass of the Lower Carboniferous strata of County 

 Dublin may be classed as compact gre}^ or darker limestone, 

 well bedded, and abounding in marine fossils. A good list of 

 these and of their localities has been published by Mr. W. H. 



1 " Geology of Ireland," p. 64. 



^ Memoir to sheets 102 and 112, Geol. Survey of Ireland, 2nd ed., p. 7. 



•''Haughton, ''Notice of the occurrence of fragments of Granite in 

 Ivimestone," G, S. D., v., 113 ; Montgomer}-, " Granite in Limestone near 

 Rathfarnham," G. S. /., i,, 15. 



