Page:Quarterly Journal of the Geological Society of London, vol. 35.djvu/859

Rh GLENGARIFF GRITS AND SLATES. 719 Silurian beds are highly discordant to the Lower; and from the ab- sence of these beds in the centre and east of Ireland, where the Lower Silurian beds reach the surface, we may conjecture, after making allowance for denudation, that these districts were land sur- faces through a portion at least of the Upper Silurian period. The physical geology of this part of the British Isles is therefore in har- mony with that which obtained in the region of Wales, Shropshire, and the borders of the Wye ; over this region, as Professor Ramsay has pointed out, the Lower Silurian and Cambrian rocks formed a land surface at the commencement of the Upper Silurian epoch, the extent of which gradually diminished by submergence until it was converted into several islets towards the close of that period*. Pos- sibly this early land surface embraced St. George's Channel, the bordering districts of Waterford, Wexford, and Wicklow, stretching into the centre of Ireland, as around the Galtees, the Silver-mine, and Commeragh mountains we find the Lower Silurian beds overlain directly by the Old Red Sandstone. The Upper Silurian basin must have extended far into the Atlantic, and have been of enormous depth in the region bordering the coast of Kerry, while towards the north it was bounded by the crystalline metamorphosed Lower Silu- rian rocks, which appear in West Galway and Mayo, and enclose the Upper Silurian trough of the Killaries and Mweelrea. It is possible that the upper portion of the Glengariff series is newer than the uppermost of the Ludlow beds. The existence of the " Upper Ludlow bone-beds " shows that the border districts of Wales were but slightly submerged at a time when the Upper Glen- gariff beds may have been deposited in deeper waters. The Upper Glengariff beds may possibly form the connecting links between the Upper Silurian and Lower Devonian series. Y. Relations or the Old Red Sandstone to the Dingle Beds, &c. Throughout the whole of the south and centre of Ireland the Old Red Sandstone is everywhere unconformable to the rocks on which it reposes, while it passes up by conformable stratification into the Carboniferous series. In the Dingle promontory it rests on various representatives of the Upper Silurian series, from the Llandovery beds upwards, in that highly discordant manner so well described by Griffith and the officers of the Geological Survey, overlapping many thousand feet of strata (see fig. 3). Great indeed have been the terrestrial disturbances and the extent of denudation in this part •of Ireland between the epoch of the deposition of the Dingle beds and that of the Old Red Sandstone. At least 12000 feet of strata have in some places been removed during this period. Again, the Old Red Sandstone wraps round tho dome-like masses of Lower Silurian beds which rise from beneath the central plain, or sometimes rises into higher elevations, crowning the heights of Galtymore and forming tho grand escarpment of tho Commeragh mountains. Traced towards the north and east through Waterford and Kilkenny, 3c2
 * Physical Geology of Great Britain, 5th edit. p. 92.