318 pkoceedijstgs of the geological society. [Jan. 18, 



worked up to the felstone ; and the underlying limestones do not for 

 some distance come to the surface. In short, we cannot but perceive 

 that when the Carboniferous series was being formed, the west side 

 of what is now Black Hill existed as a precipitous cliff- line, along the 

 base of which the deposits accumulated, and that consequently the 

 Lower Old Red Sandstones must have been tilted up and subjected 

 to an extensive denudation before the beginning of the Carboniferous 

 series in this neighbourhood. The section, fig. 1, PI. XVIII., repre- 

 sents the visible relation of the rocks in this part of the district. 



The unconformity which I wish to establish is so clearly indicated 

 in the Kerse section, that this section might be beld as decisive for 

 the entire district. But it may be well to cite another instance, 

 which occurs at a distance of twelve or thirteen miles S.W. from 

 Kerse in the parish of Muirkirk in Ayrshire. 



The Silurian shales which occupy the higher part of the Nethan 

 and Pockmair Burn cross over the ridge of hills that divides the two 

 counties of Lanark and Ayr, and stretch for five or six miles to the 

 south-west. In the neighbourhood of Priesthill the beds are inclined 

 at a gentle angle to the north-west. As they strike south-west, 

 however, the angle becomes greatly higher, and (at least in the bed 

 of the Greenock Water, below Mansfield) the beds become quite 

 vertical. This high inclination continues for rather more than a 

 mile, until, a little below the farm-house of Burnfoot, the vertical 

 Silurians and a large felstone-dyke which traverses them are over- 

 lapped by Carboniferous sandstones dipping a little S. of W., at 8°. 

 The unconformity here is of the most violent kind ; for it consists of 

 a vertical series overlaid by a nearly horizontal one. These Silurians 

 form a part of the Nethan series, since as we trace them northward 

 along tbe Greenock Water the dip lessens, and they are eventually 

 succeeded by the red shales of the lower Old Bed group. The general 

 relations of the rocks in the upper part of the Ayr Yalley are shown 

 in the section (fig. 2). 



The Cairn Table ridge, part of which is crossed by this line of 

 section, shows the same superposition of gently inclined Carboniferous 

 sandstones upon a disturbed and vertical Silurian and Lower Old Bed 

 series. 



Again, the boundary-line of the Douglas coal-field corresponds in 

 its general features to that of the Auchenheath coal-basin north of 

 Lesmahago. The general dip of the Carboniferous rocks there is 

 away from the Old Bed Sandstones, which, skirting the basin, are 

 usually inclined to the east. A fault running along the Nethan 

 Valley from the Trows to Cummerhead bas greatly disturbed the 

 Carboniferous limestones along its course ; but the connexion of the 

 different rocks, after other parts of the district have been visited, is 

 nevertheless quite apparent. If we coidd restore the strata here to 

 their normal position, we should find the same unconformity as in 

 other parts of the district ; for no sooner does the fault die away to 

 the north-east than the unconformity becomes at once apparent. At 

 Boghill and Porcheek, for instance, the Old Bed sandstones are seen 

 dipping easterly at 25° ; while immediately to the south the Carboni- 



