ment derived from the land was mingled with the calcareous and organic matter." "After the deposition of the Serpulite grit hardly any sediment derived from the land entered into the composition of the overlying limestones, and eventually nothing seems to have fallen on the sea-floor but the remains of minute organisms, whose calcareous and siliceous skeletons slowly built up the great mass of limestone and chert so conspicuously displayed at Durness." Ireland. There are five principal districts in Ireland where Ordovician rocks occur: (1) Wicklow, Wexford, and Waterford; (2) Clare and Tipperary; (3) South UlsterDown, Armagh, &c.; (4) North Ulster-Donegal and Londonderry; (5) Galway and Mayo. Between 1 and 3 are two small but important exposures, that on the Dublin coast at Portraine, and the ridge known as the Chair of Kildare. No Irish rocks have yet been identified by their fossils as contemporaneous with the Welsh Arenig. Wherever the base is seen in the east of Ireland, beds with Llandeilo fossils rest unconformably on metamorphic rocks, which are classed as Cambrian. Coast sections near Bannow and Greenore Point in Wexford expose the basement beds, which are purple conglomerates and sandstones overlain by black shales with Llandeilo fossils, a succession which recalls that of the Girvan district in Ayrshire. The Dark Shale series (Llandeilo) may have a thickness of 2,000 or 3,000 feet, and it is surmounted by an equal thickness of grey and greenish shales with interstratified igneous rocks, the slates containing fossils of Bala types. The Portraine section is remarkable for the resemblance of the rocks to the upper part of the Cumberland Ordovician system, i.e. the Coniston Limestone and the underlying Borrowdale group, with its lavas and ash-beds. There can be little doubt that this area was the centre of similar volcanic disturbances, followed by similar conditions of sedimentation, as in the case of the Cumbrian district already described (p. 32). A third centre with a similar sequence is found in the Chair of Kildare on the same line of strike, and other tracts of Ordovician rocks occur still further south-west, in the mountains of Clare and Tipperary. In South Ulster we have a continuation of the South Scottish Ordovicians, and the succession, so far as it is known, is similar; the base is not seen, but certain shales have yielded the graptolites of the Glenkiln group (Llandeilo), and others contain Bala fossils, while at Pomeroy, in Tyrone, there is a limestone from which many Bala fossils have been obtained. In Mayo and Galway there are shales which are believed to be of Llandeilo age, and others which contain the characteristic fossils of the Bala rocks; they are estimated at several thousand feet, but the stratigraphy of this part of Ireland requires further investigation. 2. Geographical Restoration. From the foregoing facts it would appear that the submergence which began in Cambrian times continued, during the formation of the Arenig series, over the whole of the western and northern portions of the British area, but that over a certain space in the centre of the sea which covered England there was an upward movement resulting in the appearance of an island composed of Archæan and Upper Cambrian rocks. In an east and west direction this island seems to have had a length of at least eighty miles, stretching from the Longmynd in Shropshire to Charnwood Forest in Leicestershire and the neighbourhood of Kettering, and possibly even further to the east. Of its northerly extension we can predicate nothing at present; 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 regarded 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 metamorphic rocks of Mayo and Galway are Archæan, and the Llandeilo shales rest upon them with an entire absence of Cambrian and Arenig beds, we may suppose that this part of the Archæan 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." He might have supported this suggestion by a reference to 1 "Address to the Roy. Phys. Soc., Edinburgh, 1886," p. 7. |