8 CARBONIFEROUS LAMELLIBRANCHIATA. 



this time was one of slow depression, this was not by any means constant ; 

 periods of intermission, or even of elevation, marked by the growth of the vege- 

 tation which formed the coal, must have supervened, and at times even the gradual 

 movements of depression must have been accentuated, causing a retrocession of 

 the shore-line, and the extension of the area of marine deposit, so that the 

 deposition of calcareous ooze was unaffected by the mixture of sediments arising 

 from the denudation of the land. The pureness of the limestone deposit may 

 have been due either to the greater distance from such areas of contamination, or to 

 the existence of Barrier reefs or other submarine ridges which would filter off the 

 suspended matter. Deposition, however, seems to have been always in the end 

 greater than the amount of depression ; for even in the most typical areas of 

 marine sediments evidence shows that there was a gradual filling up, the lime- 

 stones are gradually replaced by shales, with thin-bedded limestones, then shales 

 and sandstones, and finally the terrestrial conditions necessary for the growth of a 

 large and varied flora obtained. 



It is not improbable that the Cambrian and Granitic areas of Wexford, Carlow, 

 and "Wicklow were continuous with the similar deposits of Wales, and formed a 

 peninsula, washed on the south, north, and west by the Carboniferous sea, on the 

 bottom of which the thick deposits of Irish limestone were accumulating. The 

 Carboniferous succession of Howth, Malahide, and that neighbourhood affords 

 undoubted evidence of the close proximity of land, and represents the beds 

 deposited on the northern shore of this peninsula. Indications of a coast-line can 

 be traced north to Co. Down, where, on the present shore at Holywood, shales 

 with Modiola Macadamii occur ; similar shales also being found in Londonderry 

 and Tyrone, from where the shore-line probably passed across to the Ayrshire 

 coast. The Cumbrian area of older rocks was probably continuous with those of 

 similar age in the Isle of Man, and perhaps the Mourne Mountains, but this mass 

 must have been separated from the Welsh land by a straight or deep arm of the 

 sea, in which were deposited the limestones of North Wales, extending as far nortli 

 as the southern part of the Isle of Man. The succession of deposit in the North 

 Wales area is extremely interesting, showing the following sequence, according to 

 Prof. A. H. Green, as quoted by H. B. Woodward, ' Geology of England and Wales,' 

 p. 158: 



(Grey limestone, 

 Thm- bedded earthy limestone, 

 -b laggy and sandy Imiestone ; 

 Hard, close-grained sandstones. 

 Shales, 

 Soft fine-grained sandstones, 



which is typical of sea gradually becoming shallower. 



Millstone Grit 



AND 

 YOREDALES 



