Vol. 62.] AN UNCONFORMITY IN THE COAL-MEASURES. 531 



yellow sandstone which forms the Table Rocks, and we shall call it 

 by that name. About 40 feet of this well-marked rock is visible 

 in the cliffs of which it is the main constituent, and its total 

 thickness, as shown in sinkings inland, is not more than 50 feet. 

 It is more massive and compact in the southern parts of the section, 

 thin and irregular subordinate layers of shale developing within it 

 farther north. Its base lies unconformably upon a series of 

 alternating shales and sandstones, and it is this unconformable 

 junction which we propose to describe. 



Prom beneath the line of unconformity to the lowest tide-level, 

 about 30 to 40 feet of the shales and sandstones just mentioned 

 can be observed and measured ; but only the topmost 8 or 10 

 feet, of this series can be proved to have been subjected to the 

 denudation which preceded the deposition of the Table-Rocks 

 Sandstone, although it is natural to suppose that considerably more 

 was involved in an erosion so widespread as this. How much 

 missing rock the unconformity may represent it is not possible, with 

 the evidence at present available, to tell. 



The dip of both sets of beds — above and below the unconformity — 

 is, except where in the lower these are faulted and disturbed, one 

 of a few degrees (about 4° on the whole) to the south-west, or inland. 

 This brings about a general parallelism between the scarped 

 outcrops of the cliff-making Table-Rocks Sandstone and the hard 

 bands of sandstone of the lower series, which has, no doubt, helped 

 for many years to mask, and withdraw attention from, the uncon- 

 formity. 



Since beds of shale and sandstone (such as those of the lower 

 series) are of the kind which is common in all parts of the 

 Coal-Measures in this coalfield, and bear no special characters 

 by which they could be distinguished, it is fortunate that 

 the series includes in its upper portion a stratum of marked 

 individuality — a bed of clay-ironstone crowded with shells of 

 Carbonicola (olim Anihracosia) aquilina, which we will call the 

 Mussel-Band, a bed which serves as a most useful and unmistak- 

 able horizon of reference. 



It may be permissible to remind the reader that in the Upper 

 Carboniferous of the North of England, mussel-bands are, with the 

 possible exception of the thicker coal-seams only, quite the most 

 persistent strata met with. In consequence of their remarkable 

 constancy, they are of great use as guiding-beds in seeking for, and 

 in correlating, certain coals. If the correlation adopted in the 

 Geological Survey-map of this district be followed, then this 

 particular shelly ironstone is one well-known as occurring a little 

 way above the Low Main Seam, but this reference we are not at 

 present in a position either to combat or to confirm. Indeed, the 

 recognition of the unconformity throws many difficulties in the 

 way of the correct interpretation of inland pit- and bore-sections, 

 and doubt must (for a time at least) exist as regards several of 

 them. 



