CHAP. VI.] CARBONIFEROUS PERIOD. 73 



when the lower beds of the Carboniferous limestone were 

 formed is shown by the* overlap of this limestone on to the 

 granite of Carlow, and it is probable that this land being 

 steep and mountainous was not wholly submerged till the 

 close of the Carboniferous period, and perhaps not even 

 then. This lower limestone continues to border the older 

 rocks through Kildare, but in County Dublin it appears to 

 be overlapped by shales belonging to the Calp and Upper 

 Limestone series. 



The limestones between Howth, Swords, and Rush are 

 believed to belong to the Lower Limestone, but they show 

 unmistakable evidence of the close neighbourhood of land. 

 The Hill of Howth was clearly an island in this Lower Lime- 

 stone sea, and part of a shore conglomerate still exists 

 there. Between Kush and Skerries the limestones include 

 thick beds of conglomerate containing pebbles derived 

 from the neighbouring Ordovician rocks ; there is some 

 doubt whether these limestones belong to the Lower or 

 Upper division, 1 but there is no doubt that farther north- 

 west, near the Naul, the Upper Limestone rests against the 

 Ordovician shore, and that it is moreover overlapped by the 

 succeeding (Yoredale) shale group. At one place there 

 are boulder beds, consisting of blocks from the Ordovician 

 rocks cemented together by grey Carboniferous Limestone, 

 and Professor Jukes observes : 2 " This is evidently a portion 

 of the very beach or margin of the Carboniferous sea in 

 which the fallen blocks and shingle from the wasting land 

 above were enveloped in the calcareous deposits of the 

 Carboniferous period." 



The facts described in the memoir referred to seem ex- 

 plicable only on the supposition that the beds were deposited 

 in a bay which had land to the south, east, and north 



1 See " Mem. Geol. Survey (Ireland), Expl. of Sheets 102 and 112," 

 pp. 65, 66. 



2 Op. cit., pp. 59, 60. 



