DISLOCATIONS AND INVERSIONS. — COAL PLANTS. 



379 



After all, however, though we are here presented with singular collocations, a geologist 

 previously well acquainted with Siluria and South Wales, would have no great difficulty 

 in marking the distinctions. In other parts round the northern lip of this coal tract, 

 where it extends inland to the north of Brawdy and by Roche Bridge, the dark coloured 

 culm measures being separated from the purple Cambrian rock, by a zone of uncon- 

 formable, light coloured sandstone and millstone grit, there is an increased facility in 

 drawing the boundary. The only real difficulty, therefore, in defining the exact de- 

 marcation occurs, as before stated, where the shale of the coal measures is directly in 

 contact with the shale of the Silurian System. 



Amid the various dislocations to which these coal measures have been subjected, 

 there is no one more striking, than that which occurs along the escarpment of a thin and 

 broken zone of mountain limestone north of Johnston, where the strata of a coal-bear- 

 ing tract dip to the south, while the carboniferous limestone on the south, plunges also 

 in the same direction and therefore overlies the coal. Throughout the tract, indeed, 

 included between Johnston and Haverfordwest, the culm which is worked from the 

 natural outcrop to depths of seventeen fathoms, resembles that upon the coast in the 

 amount of disturbance, the broken and contorted masses being termed ''tumbling 

 hillocks." Small culm is the only produce, and of this there is one good seam which 

 is inters tratified with flaglike shale overlaid by sandstone, grit and shale. 



The phenomena of the inverted strata above described, is not confined to the west bank 

 of the Cleddau ; for on tracing these same culm measures on the east bank, we perceive 

 precisely the same line of disturbance extending by Langam Ferry, where the whole of 

 the productive culm measures lie in a reversed position, the younger rocks seeming 

 absolutely to underlie the older. 



Phenomena precisely analogous have been previously spoken of in Montgomeryshire 

 (p. 309.), and similar examples will be hereafter pointed out in the Abberley Hills. In 

 Pembrokeshire, indeed, as in Worcestershire, we can account for the inversion by the 

 adjacent protrusion of volcanic rock, which has heaved the older strata into this unna- 

 tural position. Here also we have the advantage, of being able to follow the dislocated 

 strata till they fall into their regular order, the culm measures resting regularly on the 

 millstone grit and mountain limestone ; and thus the extent and nature of the inversion 

 are completely explained. 



Before we take leave of the coal or culm-bearing measures of Pembroke, I may state 

 that a numerous collection of their fossil plants, chiefly procured through the exertions 

 of my valued friend, Mr. Leach of Milford Haven, has been examined by Professor 

 Lindley, who is of opinion, that all these plants occur in other coal fields. They con- 

 sist of various Lepidodendra and Calamites, most of which, from their fractured condition, 

 are indeterminable, together with the common coal plants Neuropteris gigantea, Peco- 

 pteris conchitica, P. nervosa, Sphenophyllum Schlotheimii, Stigmariajicoides, &c, &c. These 

 are abundant in the Salopian coal measures, which occupy the same place in the 



