proposed separate names for these basins, calling the south-eastern area Lake Cheviot, the central one Lake Caledonia, the small western basin Lake of Lorne, and the northern basin Lake Orcadie. From the proofs which have been adduced of the original wide extension of the Old Red Sandstone (p. 52), it might be thought that the three principal basins could hardly have been separate lakes, but must have been inlets proceeding from one large inland sea, the greater part of which lay to the east of Scotland; and, indeed, so far as the stratigraphical evidence goes this would be the most natural conclusion, for the lithological differences between the strata of the several basins are hardly greater than the differences which exist between the Lanark and Forfar types in the same Caledonian basin. The paleontological differences are, however, very much greater, the piscine fauna of the Forfar and Caithness flags being so distinct that Sir R. Murchison thought they could not be of the same age, and was led to suggest that the Caithness flags formed a middle group distinct from the Lower Old Red, and of younger date than the flags of Arbroath in Forfar. Dr. Geikie has shown that this is improbable, and that the discrepancy is not complete, and that the general succession of beds in the two areas is very similar. "The admitted palæontological distinctions are probably not greater than the striking lithological differences between the strata of the two regions would account for, or than the contrast between the ichthyic faunas of contiguous water-basins at the present time."1 The difference is, however, sufficiently remarkable, for out of eighteen genera with sixty species from Caithness, and ten genera with seventeen species from Arbroath, only four genera and one or two species are common to the two areas. So great a difference, though it may perhaps be lessened by future 1 "Textbook of Geology," first edition, p. 715. discoveries in Forfar, affords good ground for concluding that there was no direct communication between the waters of the two lakes. There is less evidence for regarding the Cheviot basin as distinct from the Caledonian, as no fish have yet been found in the Cheviot district, and we do not know how far the Lower Old Red originally stretched over the southern uplands; much of it was removed before and during the formation of the Upper Old Red, the latter in all probability deriving much of its material from the destruction of the older series. The Lower Old Red partakes in the plication of the Silurian and Ordovician rocks of southern Scotland, and it may for ought we know have been coextensive with them over the whole region. On the other hand, it should be noted that the Lower Old Red of the Cheviot district rests unconformably on the Silurian, which is an unusual relation, and suggests that the Cheviot basin was formed at a somewhat later date than the Caledonian basin by a local depression of the land surface. This, however, is not incompatible with its having then been an inlet or extension of the Caledonian Lake, but I have not ventured to express this view on the map, which is a rendering of Dr. Geikie's opinion. The greater part of Ireland likewise seems to have been land enclosing lake basins, and was doubtless at this time connected with England and Scotland, so that the whole formed one mass of land, and was part of a large continent that extended far westward into the place now occupied by the North Atlantic. The tract of Old Red Sandstone in Tyrone being in alignment with that of the central Scottish basin, Professor Hull has suggested that the Lake Caledonia extended thus far into Ireland, and I have adopted this view in the restoration of Old Red Sandstone geography (Pl. III.). The other small tract of Old Red recently discovered in Donegal may have been formed in a separate basin, which has been named Lake Fanad by Professor Hull. We may now glance at the succession of events in the south of England.. Here, in the sea which occupied the space between the northern part of France and the Bristol channel, there had been continuous deposition of sediment throughout the Devonian period. After the first elevation at the close of Silurian times, this southern area does not seem to have been affected by the upheaval which was in progress to the northward, and indeed it may have occupied a trough of compensating depression. The western part of it was raised into land, however, after the formation of the Glengariff Grits, all the south of Ireland probably being land during the formation of the Middle Devonian rocks. According to Professor Gosselet, the Devonians of the Boulonnais and the Ardennes were deposited in a narrow strait connecting the wider seas of the Westphalian and the Anglo-French areas. Along the northern side of the Devonian tract from Boulogne to Namur the Lower Devonian is absent, and the base of the Eifelien division consists of conglomerate and sandstone. It is highly probable that the land thus indicated was connected with that which lay over the centre and east of England. Parts of Brittany and Normandy seem also to have been land, and Professor de Lapparent thinks that they formed an island, the limits of which were very nearly the same as those of the present massif of Cambrian and Ordovician rocks. Inlets of the Lower Devonian sea penetrated the district near Brest, in Basse Loire, on the frontiers of Maine and from the Normandy side; but whether it sank beneath the sea of the Middle and Upper Devonian epochs is not known, as the strata of these stages occur only to the south in Basse Loire. 1 "Esquisse géologique du nord de la France,” p. 60; but the evidence for the southern shore of this strait is inconclusive. |