46 PALEOZOIC TIME. [CHAP. IV. 



a promontory jutting out from a main line of coast, but I 

 have not ventured to insert either on the map (PI. I.), as so 

 little is known of their lateral extent, and as yet they have 

 only been proved to occur over a small area. To the east 

 there was open sea, and the close correspondence between 

 the Scottish and Swedish zones of life leads us to infer 

 that the sea-bed was continuous from one region to the 

 other. 



It is possible, however, that an island of considerable 

 size lay between Ireland and Wales, and included certain 

 areas which now form portions of these countries. The 

 lateral change which takes place in the character of the 

 Silurian strata as they pass westward from Shropshire, 

 and the thickening of the sandstones to the north-west, are 

 facts which indicate land in that direction. From the 

 quantity of felspar grains that enter into the composition 

 of the Denbigh grits, Sir A. Ramsay has inferred " that 

 part of the mountain region of North Wales between 

 Conway and Cader Idris then formed land (very different 

 from its present form), which in some degree contributed 

 locally to make these strata by the waste of the felspathic 

 lavas and ashes that form such a distinguishing feature of 

 the Lower Silurian rocks." ] If, however, the Ordovician 

 tracts of eastern Ireland were united to those of Wales, 

 and formed a continuous mass of land during part and, 

 perhaps, the whole of the Silurian period, there is no neces- 

 sity to suppose that the felspathic material was derived 

 from the Snowdon region in particular, since felspathic 

 lavas occur also in Wicklow and Wexford, and were doubt- 

 less exposed at many places over the intervening tract of 

 land. Northward this land may have extended as far as 

 the Isle of Man, and the conglomerates which occur in the 

 Sedburgh and Settle districts suggest the presence of 

 islands at its north-eastern end, but there can be little 

 1 " Geology of North Wales," second edition, p. 285. 



