378 



ANOMALOUS POSITIONS OF CULM MEASURES EXPLAINED. 



certained by observing their order of superposition, we may, as I have already said, ge- 

 nerally assure ourselves of the real age of such beds, when we find them void of Silurian 

 organic remains, and still more easily, when we detect true coal plants in them. There 

 is also another test, which though it may be empirical and unsafe as applied to larger 

 tracts of country, is true as respects Pembrokeshire, viz., the much more abundant 

 presence of iron, both disseminated and in smaller impure concretions than is found in 

 the Silurian rocks. In this district, however, we have no need of such tests, for we can 

 safely determine the age of these strata by following them up to a well-defined base 

 line. 



Thin coal seams, indeed, begin to appear even in these beds of Druson Haven, for- 

 merly called greywacke, and thence in the ascending order, culm is irregularly distri- 

 buted throughout a considerable thickness of sandstone and shale, most of the beds, 

 having after all, a very different lithological character from that of the Upper Silurian 

 Rocks. 



The other case above alluded to, is where the culm measures appear to pass down- 

 wards into the Cambrian Rocks. This occurs about half a mile north of the little 

 stream which empties itself into the sea, by passing through the great shingle bank at 

 Newgale Bridge 1 . Viewing the cliffs from the shore, it is no easy matter to define where 

 the older strata cease and the younger begin, though on the one side of the depression 

 are the Old Cambrian Rocks which range to St. Davids, on the other the true coal 

 measures. 



When the stratified masses of these two systems, which form the fringe of this bold 

 coast, are looked at from the sea, or from the strand at low water, an artist, indeed, 

 would naturally sketch them as lying conformably and dipping to the east of south at 

 angles of about 40°. A gross error, however, would be committed in assimilating them, 

 for on close examination the apparent conformity vanishes, and the mineral characters of 

 the two classes of deposit is also seen to be distinct. To the north, the beds consist of 

 purple, finely laminated, hard sandstone, like that of the Longmynd in Salop ; to the south 

 lies the millstone grit, consisting of grey sandstone, with some ferruginous matter and 

 iron-stone concretions, a part of the carboniferous strata which dips under the productive 

 culm strata of Newgale, Brawdy and Eweny. Further, the purple greywacke when 

 accurately observed, is not strictly conformable to the coal grits ; there being a percep- 

 tible discrepancy of strike (about 10° to 15°) between the one and the other, though they 

 both have so nearly the same inclination as to appear conformable. Now if this junc- 

 tion were not exposed in a bold sea cliff, where the faces of these rocks are completely 

 laid bare, how much might have been written upon conformability and passage ! and 

 what erroneous inductions might have been drawn from these fallacious appearances ! 



1 The actual point of junction in the shore is marked by a depression in the cliffs. A small dyke or boss of 

 trap is laid bare at low water, and partially alters and distorts the lowest of the carboniferous beds, which are 

 hardened and highly ferruginous. 



