Silurian Rocks of the South of Ireland to those of North Devon. 149 
less in the centre and east of Ireland, where the Upper Silurian beds, including 
the Glengariff grits and slates, are absent. It is impossible to say to what extent 
this is due to denudation rather than to absence of deposition. <A similar process 
of subsidence took place over the Silurian area on the Welsh borders—the North 
Welsh Highlands being a portion of the land of the period. 
At the close of the period to which the Glengariff beds belong, the rocks over 
the region of the south-west of Ireland were disturbed and elevated into land, and 
so exposed to denudation ; owing to which they are in a position of discordaney (as 
we have seen) to the formations which succeed them. Hence the hiatus in that 
region. It was otherwise, however, over the area of the south of England and 
Wales. There continuous depression appears to have gone on after the close of 
the Upper Silurian period, and during this depression the Lower and Middle 
Devonian rocks were formed in a sea thickly peopled by various forms of life 
over the Devonian area. Over the Welsh border area esturine conditions 
prevailed, owing to special physical causes which Sir H. 8. De la Beche and 
Professor Ramsay have well explained. In this physical condition of the 
region of the south of Ireland we have a satisfactory explanation of the entire 
absence of the Lower and Middle Devonian beds, and it was only upon the com- 
mencement of the Upper Devonian (or Old Red Sandstone) epoch that the area of 
the south of Ireland partook of the general depression, and the lower flanks of 
the hills together with the plains were submerged. 
As regards the submergence of the south of Ireland and Devonshire at this 
epoch, it is probable that the waters were esturine or lacustrine, as the presence of 
the fresh-water mussel, Anodonta Jukesi, in the upper beds of the Old Red Sand- 
stone evinces conditions of this kind. But, over the Continental area, the conditiong 
were probably marine, as the ‘‘ Psamite du Condroz,” which is the representative 
of the Pickwell Down Sandstone of Devonshire, and of the Old Red Sandstone 
of the south of Ireland is certainly a marine formation. The Jacustrine conditions 
ultimately gave place to those of a marine character at the commencement of 
the Carboniferous stage, during which the area of the British Isles was con- 
tinuously depressed, and the sea spread over the whole region, with the exception 
of a few elevated tracts which stood up as islands.* The Carboniferous materials 
were piled over the slowly subsiding sea-bed, climbing up along the flanks of the 
higher elevations as they were successively inundated ; and it is not improbable 
that, at the close of the epoch of the Carboniferous Limestone, all the Upper 
and Lower Silurian hills were enveloped in Carboniferous strata. This gradual 
overlap of the Carboniferous beds during the submergence of the shelving-shore of 
Glengariff Beds is illustrated in the following woodcut (Fig. 6). 
* In the Coal-fields of Great Britain, 3rd Edition, I have endeavoured to show on a map of the British 
Isles the submerged and land areas at the beginning of the coal period. 
