The great fault crossing Scotland from near Dunbar to the Ayrshire coast, which separates the Silurians of the South of Scotland from the Old Red Sandstone and Carboniferous tracts of the North, has been found by Mr. B. N. Peach, of the Geological Survey,[38] to have in some places a throw of fully 15,000 feet. This great dislocation is older than the Carboniferous period, as is shown by the entire absence of any Old Red Sandstone on the south side of the fault, and by the occurrence of the Carboniferous Limestone and Coal-measures lying directly on the Silurian rocks. We obtain here some idea of the enormous amount of denudation which must have taken place during a comparatively limited geological epoch. So vast a thickness of Old Red Sandstone could not, as Mr. Peach remarks, “have ended originally where the fault now is, but must have swept southwards over the Lower Silurian uplands. Yet these thousands of feet of sandstones, conglomerates, lavas, and tuffs were so completely removed from the south side of the fault previous to the deposition of the Carboniferous Limestone series and the Coal-measures, that not a fragment of them is anywhere to be seen between these latter formations and the old Silurian floor.”[39] This enormous thickness of nearly three miles of Old Red Sandstone must have been carried away during the period which intervened between the deposition of the lower members of the Lower Old Red Sandstone and the accumulation of the Carboniferous Limestone. Near Tipperary, in the south of Ireland, there is a dislocation of the strata of not less than 4,000 feet,[40] which brings down the Coal-measures against the Silurian rocks. Here 1,000 feet of Old Red Sandstone, 3,000 feet of Carboniferous Limestone, and 800 feet of Coal-measures have been removed by denudation off the Silurian rocks. Not only has this immense thickness of beds been carried away, but the Silurian itself on which they rested has been eaten down in some places into deep valleys several hundreds of feet below the surface on which the Old Red Sandstone rested. [Footnote 34: _Mem. Geol. Survey of Lancashire_, 1862.] [Footnote 35: _Mem. Geol. Survey of Great Britain_, vol. iii.] [Footnote 36: _Memoir_ to Sheet 32, Geol. Survey Map of Scotland.] [Footnote 37: _Nature_, vol. xiii. p. 390.] [Footnote 38: _Explanation_ to Sheet 15, Geol. Survey Map of Scotland.]