CHAP. III.] OEDOVICIA.N PEBIOD. 37 



it is not at all unlikely to have stretched some distance 

 to the north-east, but I have not ventured to indicate such 

 an extension on the map. Southward it probably sent a 

 promontory as far as Malvern, but its southern boundary 

 is as uncertain as its northern limit. 



West of this island there seems to have been another 

 formed by the unsubmerged portion of the ridge spoken of 

 in the last chapter as reaching across from Anglesey to the 

 south-east of Ireland, but how much farther to the west 

 and south it extended at this time we have no means of 

 knowing, so that the tract shown on the map may be re- 

 garded as the minimum amount of land in this region. 



It is difficult to say how much of Ireland may have been 

 land in Arenig times, because we do not yet know whether 

 any Arenig beds exist in that country or not, but they 

 certainly seem to be absent in Wicklow and Wexford, and 

 they have not been identified elsewhere. If the meta- 

 morphic rocks of Mayo and Galway are Archaean, and the 

 Llandeilo shales rest upon them with an entire absence of 

 Cambrian and Arenig beds, we may suppose that this part 

 of the Archaean surface was not submerged till after the 

 beginning of the Ordovician period. How far this land 

 extended eastward in Upper Cambrian and Arenig times 

 is a point of very great interest when viewed in connection 

 with the great differences which exist between the Arenig 

 rocks of England and those of northern Scotland. 



The similarity of the Durness and Trenton limestones 

 and the American aspect of the fauna found in the former 

 have suggested to Mr. Peach that " some old shore-line or 

 shallow sea must have stretched across the North Atlantic 

 or Arctic Oceans, along which the forms migrated from one 

 province to the other, and that some barrier must have cut 

 off this area from that of Wales and central Europe." l 

 He might have supported this suggestion by a reference to 

 1 " Address to the Roy. Phys. Soc., Edinburgh, 1886," p. 7. 



