Between the thick and thoroughly marine development of the Carboniferous Limestone in Derbyshire and in South Wales, there lies the region, already referred to, wherein both the Carboniferous Limestone and Millstone Grit die out against what must have been a ridge of land or group of islands that stretched in a general east and west direction from the high grounds of Wales through Shropshire, Staffordshire and Leicestershire. On the slopes of this ridge the limestone is gradually overlapped by the Millstone Grit, and both are in turn overlapped by the Coal-measures, which are then found lying immediately on the more ancient rocks of the region--Cambrian or pre-Cambrian, Silurian and Old Red Sandstone. The gradual subsidence that led to the deposit of several thousand feet of Carboniferous strata over the regions to north and south, before the beginning of the Coal-measure period, does not seem to have sensibly affected the persistence of this old terrestrial surface, which probably lay on an axis of upward movement, so that, amidst the surrounding depression, its position above water was on the whole maintained. But there are indications that the inequality of movement in this part of the earth's crust was of much older date than the Carboniferous period. The Old Red Sandstone is conformably continuous below the base of the Carboniferous system, and in Wales is estimated to be some 10,000 feet thick. No break has yet been detected in this vast accumulation of sedimentary material, though it is highly probable that some such unconformability must exist in it as that between the Scottish Lower Old Red Sandstone, which passes down into the Upper Silurian shales, and Upper Old Red Sandstone, which graduates upward into the base of the Carboniferous formations. But even if such a break should be discovered, it will not account for the position of the Coal-measures on Cambrian or even perhaps older rocks. It is hardly conceivable that, had these rocks been covered with a full development of Old Red Sandstone, they could have been stripped of it by denudation before the deposition of the Coal-measures. It seems much more probable that the discrepancy in the terrestrial movements had commenced in Old Red Sandstone time, and that these ridges of ancient Palæozoic rocks never sank below the waters in which the vast thickness of red sandstones, marls and conglomerates was laid down.[112] [Footnote 112: See a discussion of this subject in Jukes' Preface to his _South Staffordshire Coal-field_.]