The middle portion of the D1 beds seen in the Gratton Dale section consists of about 630 feet of dark dolomitic limestones, which appear to be confined to a definite horizon in the sequence. The impression one at first receives is that here are definitely bedded dolomites probably of contemporaneous origin, and this material has already been referred to as a thick bed of dolomite. Working northwards down the dale, i.e. ascending the sequence, one finds that the vertical passage from normal limestone into dolomitic material above is everywhere obscured. Similarly the upper limit of the dolomitized beds is covered, so that it is impossible to say whether the metasomatism abruptly ceases with a certain bed or dies out gradually in the higher strata. In the affected part of the sequence the dolomitization appears persistent in Gratton Dale, and at a few places only have I found lateral transitions, one or two examples on either side of the dale. The most marked transition is on the western side, towards the upper part of the altered material. Here light-coloured limestones are seen to pass laterally within a few yards into darker dolomitic material, with which it contrasts. Careful examination shows that these limestones on both sides of the Dale are really transitions, and are not interbedded with dolomite, a matter of considerable significance. North-westwards the dolomitization extends for some distance up Long Dale which joins Gratton Dale at right angles.