of it. The older beds are only found in the central part of the area, and the water was not clear enough for the formation of limestone during the whole time, the higher beds being chiefly dark earthy shales, which overlap the limestones on to the sinking land both northward and southward, as doubtless they also did to the eastward. The great development of shales in this district may have been caused, as Jukes suggested, either by the influx of a river which had previously some other debouchure, or by the sea having reached some tracts of earthy Ordovician shale which had previously been above its level (op. cit., p. 23). North of the area just described, and around the town of Drogheda, is another tract of Lower Carboniferous strata lying in a hollow between ridges of Ordovician rocks. The Lower Limestone is only found to the westward, and disappears about two miles east of Slane, owing "to the conformable overlap of the higher beds of the formation on to the Silurian (i.e. Ordovician) rocks, over which they were deposited along a gradually shelving shore." " This tract was probably, therefore, another bay narrowing eastward. The tract of Carboniferous beds between Ardee and Kingscourt exhibits a complete section of the Lower series, and these beds doubtless extended originally all over the Ordovician tracts of Cavan, Monaghan, and Louth. There are also outlying tracts near Dundalk and Carlingford, but these seem to belong to the Upper Limestone series, and it is not unlikely that a large part of County Down was a promontory of land jutting westward into the sea of the Carboniferous Limestone. In the north-east of County Down, at Holywood and Castle Espie, there are limestones associated with red shales and sandstones, which are probably shore-beds of Upper Limestone age. 1 "Mem. Geol. Surv. (Ireland), Expl. Sh. 91 and 92,” p. 35. We have now traced indications of the existence of a coast-line all the way from Wexford and Waterford to the north of County Down, and here we are, doubtless, not far from its northern limit, for there can be little doubt that there was a connection between the Irish and Scotch waters at this period. The rocks of County Down are evidently a continuation of the Silurian and Ordovician tracts of southern Scotland, and it is highly probable that in early Carboniferous times they formed one continuous mass of land, and it is not at all improbable that this land included the northern part of the Isle of Man. It is true that there is carboniferous limestone at the southern end of the Isle of Man, but there is no proof that it ever extended far to the north, and the red sandstones and cornstones of Peel may be shore-beds of the Limestone sea, like the red sandstones of Scotland. That this was the case on the northern border of the land area we are now considering is the decided opinion of the Geological Surveyors of Lanarkshire. Describing the basal conglomerates of this district, they say: "These conglomerates continue to fringe the Carboniferous area, while the strata above pass quite away. Hence, in this continuous band of conglomerate, one portion is on the horizon of a low part of the Calciferous Sandstone series, while another portion is on the horizon of the Carboniferous Limestone series. It thus brings before us evidence of shore conditions during a protracted submergence of this area in Lower Carboniferous times."1 It must be remembered, also, that what is here called the Calciferous Sandstone is now regarded as contemporaneous with the lower part of the English Carboniferous Limestone. Returning to the Isle of Man, the existence of the Limestone series there is no proof that there was open sea to the 1 "Mem. Geol. Surv. of Scot., Expl. Sh. 15," p. 30. westward. It is, of course, possible that there was communication by way of a narrow strait between the Carlingford area and that of the Isle of Man, but the evidence on the Irish side is in favour of there having been a continuous mass of land over the western part of the Irish Sea, and the Manx rocks are just as likely to have been formed in a bay on the eastern side of this land, seeing that there were several such bays on the western side. Exactly the same reasoning applies to the Carboniferous rocks of Anglesey. The succession here is very similar to that of the Isle of Man-a basal conglomerate, succeeded by red sandstones and cornstones from 200 to 300 feet thick, overlain by limestones only 450 feet thick, and covered directly by the Millstone Grit. These small thicknesses suggest the neighbourhood of land, and that this lay to the west is shown by the fact of the Limestone series rapidly thickening to the east, and attaining some 2,000 feet in the north of Flint. It is quite possible, therefore, that the western part of Anglesey was land, and that the Carboniferous beds were deposited in a bay or inlet which penetrated into this land, and narrowed south-westward. Patches of limestone skirt the coasts of North Wales and border the Silurian rocks along the Vale of Clwyd; here and in North Flint its thickness is about 1,500 feet, and it keeps this thickness for some distance southward, being still 1,200 feet thick at the north-west end of the Eglwyseg escarpment near Llangollen, where it rests on 300 feet of yellow sandstone and conglomerate. Thence, however, it thins very rapidly to the south-east, being only 600 feet thick at Trevor, and under 200 feet at Fron-yCysyllte on the south side of the Dee; thus, in a distance of four miles, the red sandstones and about 1,000 feet of the limestone have thinned out against a slope of Silurian rock, a fact which suggests the existence of an island in the sea of the Lower Limestone near Ruabon and Chirk. 1 Mr. G. H. Morton 1 has shown that in this district the Limestone is divisible into four stages, and that at Fron only the highest, and 28 feet of the third remain. Near Chirk the latter have thinned out, and only the uppermost grey beds (137 feet thick) are found. That this thinning indicates an island is proved by the white beds coming in again below the grey at Craig Sychdin, seven miles south of Fron, and these continue to form a base as far as Crickheath Hill, when shales belonging to the second stage appear, and at Llanymynech the total thickness of the limestone has increased again to 450 feet. Here the escarpment terminates; and when Carboniferous strata set in again five miles to the south-east, Coal-measures rest on Silurian, so that the limestones had thinned out in the interval. With regard to the westerly extension of the Limestone we are furnished with valuable testimony in the shape of an outlier, faulted down against Silurian shales, near Corwen, and no less than twelve miles W.S.W. of the Eglwyseg escarpment. Moreover, the thickness here is still considerable, probably about 750 feet, so that the limestone must have extended some distance farther to the west and south of Corwen. It is hardly likely, however, to have reached so far as the Arenig mountains, though it may have run up the valley of the Dee as far as Bala; the northern flank of the Berwyn mountains probably formed its southern boundary, these mountains forming a promontory which stretched north-eastward towards Llangollen and Chirk, and separated what may be called the Corwen bay from the Llanymynech bay (see map, fig. 2). The absence of the limestone over the Shrewsbury district indicates another extension of the land area between Llanymynech and Wellington, where the border of the 1 "The Carboniferous Limestone and Cefn-y-Fedw Sandstone," 1879. |