OLD RED SANDSTONE, CONGLOMERATES, ETC. 



387 



/. Argillaceous, thin-bedded, light-coloured sandstone, very slightly micaceous with a profusion of casts of fossils, of which 

 notice will be taken in treating of the Silurian rocks. 



This section is quoted, to show the place of the lower conglomerate (c), which, although inter- 

 polated in the Old Red System, is perfectly undistinguishable from some of the oldest coarse grits 

 of the Cambrian System 3 for the description of the concretionary masses and beds of quartz grit 

 west of Llandovery, and extending thence to Rhayder in the heart of the old slaty rocks, might well 

 be substituted for this bed, either of them being perfect specimens of German " greywacke." This 

 circumstance is specially insisted upon, to demonstrate the absurdity of retaining this word in geo- 

 logical nomenclature 1 . 



Conglomerates like this of Canaston Wood, occur in vast quantities upon the surface of the high 

 and sterile tract north-east of Tavern Spite, near the junction with the Silurian rocks. They are 

 also seen at Rushford, north of Hoton, where the Silurian rocks protrude, and again at several 

 points in the southern or Pembroke promontory. They are for the most part easily separated from 

 a much coarser and more angular, grey-coloured conglomerate, which occurs occasionally where 

 certain Silurian rocks have been protruded. The latter is not underlaid by a great thickness of red 

 shale or rab, like the true conglomerate of the Old Red, but forms a part of the fossiliferous strata 

 of the Silurian System. The other situations where the Old Red Sandstone may be observed in- 

 cumbent on the Silurian rocks, are in the coast cliffs of Freshwater East and Freshwater West, 

 and here again the two sorts of conglomerate occur, the one in the Silurian rocks along the line of 

 fault, the other in the Old Red. But the most instructive section of a complete passage from the 

 one system into the other is at Hook Point, the southern headland of Marloes Bay. (PI. 35. f. 10. 

 and p. 392.) Here the same honey-combed, sub-concretionary beds, with red rab and sandstone, 

 pass down into and alternate with grey beds, until they are fairly dovetailed into strata of unequi- 

 vocal Ludlow rock, the top of the Silurian System. The absence of conglomerates in these beds 

 of passage, in so complete a section as that of Marloes, harmonizing with the regular sequence of 

 the same beds near Tavern Spite, teaches, that the conglomerates must have resulted from partial 

 dislocations, and as some are seen within the Silurian System and others in the Old Red, they 

 indicate the occurrence of disturbances at different intervals. It is obvious, however, that such con- 

 glomerates may have been formed by ancient tides and currents. But in whatever manner accumu- 

 lated, whether by torrents, resulting from elevations and dislocation, or caused by diurnal action, it 

 is evident that the old conglomerates in question were purely local. 



No organic remains have yet been detected in the Old Red Sandstone of Pembroke- 

 shire. This is perhaps to be expected, as it contains scarcely any of those beds of true 

 cornstone or calcareous flags in which the greater part of such remains, particularly the 

 fishes, are found. The absence of calcareous matter is, indeed, the great cause, why 

 some tracts of this red rab do not afford so warm and rich a soil as the mellow lands of 

 Herefordshire 9 . 



1 If I were disposed to extend the application of these remarks, I might I believe assert that rock specimens, 

 which many mineralogists would term greywacke, may be found in every stage of the geological series, even 

 in the tertiary deposits. Geologists who have worked out the general relations of a country are of course 

 exempted from this remark. 



3 Speaking of some of his red land, near Wollaston Green, a farmer observed to me, " it eats all the manure 

 and drinks all the water :" although this homely remark was very descriptive of the district in question, there 

 are wide tracts where the red rab decomposes to excellent land, perhaps the best in Pembrokeshire. 



