74 PALEOZOIC TIME. [CHAP. VI. 



of it. The older beds are only found in the central part 

 of the area, and the water was n6t clear enough for the 

 formation of limestone during the whole time, the higher 

 beds being chiefly dark earthy shales, which overlap the 

 limestones on to the sinking land both northward and 

 southward, as doubtless they also did to the eastward. 

 The great development of shales in this district may have 

 been caused, as Jukes suggested, either by the influx of 

 a river which had previously some other debouchure, or 

 by the sea having reached some tracts of earthy Ordovician 

 shale which had previously been above its level (op. cit., 

 p. 23). 



North of the area just described, and around the town 

 of Drogheda, is another tract of Lower Carboniferous strata 

 lying in a hollow between ridges of Ordovician rocks. The 

 Lower Limestone is only found to the westward, and dis- 

 appears about two miles east of Slane, owing " to the con- 

 formable overlap of the higher beds of the formation on to 

 the Silurian (i.e. Ordovician) rocks, over which they were 

 deposited along a gradually shelving shore." This tract 

 was probably, therefore, another bay narrowing eastward. 



The tract of Carboniferous beds between Ardee and 

 Kingscourt exhibits a complete section of the Lower series, 

 and these beds doubtless extended originally all over the 

 Ordovician tracts of Cavan, Monaghan, and Louth. There 

 are also outlying tracts near Dundalk and Carlingford, but 

 these seem to belong to the Upper Limestone series, and it 

 is not unlikely that a large part of County Down was a 

 promontory of land jutting westward into the sea of the 

 Carboniferous Limestone. In the north-east of County 

 Down, at Holywood and Castle Espie, there are limestones 

 associated with red shales and sandstones, which are pro- 

 bably shore-beds of Upper Limestone age. 



1 " Mem. Geol. Snrv. (Ireland), Expl. Sh. 91 and 92," p. 35. 



