44 PALEOZOIC TIME. [CHAP. IV. 



is any area of Silurian rock known to exist in northern 

 Scotland. So far, therefore, as we can judge from present 

 evidence, the Silurian rocks probably die out beneath the 

 Lower Old Red on the south side of the great boundary 

 fault. 



Passing now to Ireland, we find a significant absence of 

 Silurian rocks in the south-east of that country. The Ordo- 

 vician rocks of Dublin, Wicklow, and Wexford are flanked 

 by Upper Old Eed Sandstone and Carboniferous rocks a 

 sequence suggestive of these districts having been land 

 throughout the periods of the Silurian and Lower Old Eed 

 Sandstone. Rocks representing the Llandovery group occur 

 at several localities in eastern and central Ireland, as in 

 County Down, at Portraine in Dublin, and at the Chair of 

 Kildare, but the only complete sections of Silurian rocks 

 are found on the extreme western shores. In G-alway and 

 Mayo there is striking evidence of great physical changes 

 having taken place in the interval between the formation 

 of the Ordovician and Silurian rocks of Ireland. Here the 

 basement beds with Llandovery fossils rest on the edges of 

 upturned and metamorphosed Ordovician strata, so that 

 great terrestrial disturbances must have taken place, re- 

 sulting in the compression, metamorphisin, and upheaval 

 of the older series before the deposition of the Silurian 

 deposits, for the conglomerates contain fragments of these 

 underlying metamorphosed Ordovicians. 



We may reasonably infer that the existence of the conti- 

 nental land which was postulated as existing to the north- 

 west of Ireland in Ordovician times had much to do with 

 the location of this pressure and metamorphism, and that 

 the terrestrial forces finally expended themselves in an 

 elevation and extension of the land which had previously 

 existed, bringing its boundaries farther south and within 

 the limits of what is now Irish soil. During subsequent 

 subsidence the coast-line again receded northward, and the 



