LAKES IN RELATION TO GEOLOGICAL FEATURES 449 recognised this divergence as represented in Caithness, and referred the flagstone series to the middle division of the Old Red Sandstone— a view which has been strengthened by the researches of Dr Traquair in recent years. In that county, conglomerates and sandstones occur at the base, and graduate upwards into grey and blue bituminous flagstones charged with fish-remains, and succeeded by the sandstones and flags of John o' Groats. On the other hand, Sir A. Geikie contends that the Orcadian series north of the Grampians are the equivalents in time of the Lower Old Red deposits south of that chain. He further holds that the admitted palaeontological distinctions between the two areas are probably not greater than the contrast between the ichthyic faunas of adjacent but disconnected water basins at the present time. The Upper Old Red Sandstone everywhere rests unconformably on the older rocks, but graduates upwards into the Carboniferous forma- tion. The nature of this unconformability clearly shows that the members of the lower division, including the volcanic rocks, were elevated, folded, and subjected to extensive denudation before the deposition of the Upper Old Red strata. In places valleys or hollows were excavated in the Silurian tableland during this period of erosion. The strata consist of conglomerates, sandstones, marls, and corn- stones, from which a characteristic assemblage of fish-remains has been obtained. North of the Grampians they appear in the coastal belt of Elgin and Nairn, in Caithness, in the island of Hoy, and in Shet- land. They usually form a fringe round the basal beds of the Carboniferous system in the Central Lowlands and along the southern flanks of the Silurian tableland. CARBONIFEROUS The records of the Carboniferous formation are of great im- portance. The succession of sandstones, shales, limestones, coals, and ironstones composing this system have been carefully studied, owing partly to their economic value, and partly to the rich fauna and flora embedded in the rocks. Scotland possesses a large development of these rocks, though owing to subsequent folding and denudation they have been confined mainly to the Central Lowlands and the Border territory. The detailed examination of the Carboniferous areas shows that the strata are arranged in a series of basins much intersected by faults ; the crests of the anticlinal folds being occupied by the lower subdivisions of the formation, or by rocks of older date. One of the best examples of this disposition is the great Lanarkshire basin, which is bounded on the north by the Campsie Fells, on the west by the Renfrewshire and Eaglesham Hills, on the south by the Old Red Sandstone of Lesmahagow and Lanark, and on the east by 29.