TRITURATED HEAPS OF CULM CAUSED BY CONVULSIONS. 377 



the convulsions and contortions of the strata, cavities or " slashes " were here and there 

 produced, whilst the culm which fills them, naturally very frangible in its original state 

 of bedded coal, would be ground to small particles by the lateral pressure. These re- 

 lations are expressed in this wood-cut. 



75. 



a. Slash of finely triturated culm between contorted culminiferous strata. b. Slough, i. e. vertical beds of coal, 

 c. One of the innumerable faults of this coast. The slash (a) is supposed to be upon a line of violent fracture. 



At whatever period formed, it is evident these " slashes " have been derived from 

 the wreck or breaking up of some of the regular coal seams, the ends of which are 

 usually to be met with in the proximity of these small basins. We may here remark, 

 that nothing can better convey a conception of the extent of force, employed in 

 fracturing masses after their consolidation, than the finely comminuted slate of the 

 coal, which from its frangible nature was shivered into small fragments, whilst the solid 

 ribs of rock were undergoing those terrific movements which left them in their present 

 contorted positions. The above wood-cut will convey some notion of the manner in 

 which the beds are thrown about in the adjoining coast cliffs. 



Reverting to what I have already stated concerning the rocks on which the carboni- 

 ferous strata of Pembrokeshire repose, I refer my readers to the 5th and following 

 chapters, in which they will perceive, that like these, the Salopian coal measures 

 repose on rocks of all ages, from the mountain limestone to the Cambrian rocks in- 

 clusive. This collocation, which in Shropshire cannot lead us into error, has been 

 productive of confusion in those situations where the coal measures put on the litho- 

 logical characters of the older deposits, and at the same time rest directly upon them : 

 and if there is no striking want of conformity between these masses, their separation 

 becomes a subject of difficulty, to persons not habituated to such phenomena. 



Thus, for example, to the east of Nolton, it would be difficult, as before remarked, 

 for the closest observer to define the boundary between the culm beds and the Lower 

 Silurian shale and sandstone, for there is a striking coincidence in the lithological aspect 

 of the two rocks, and very little apparent discrepancy in their position. It is only by 

 detecting here and there in the lower strata, a fossil which we know to be peculiar to 

 the Caradoc sandstone, &c, and observing in the upper beds, fragments of coal plants, 

 that we can convince ourselves of the separation. 



In fact, the lowest strata of the culm measures of Pembroke consist of flaglike sand- 

 stones, alternating with dark grey beds of hard and soft shale. The latter, where not 

 micaceous, are to an unpractised eye, scarcely to be distinguished from many beds of 

 the Silurian System, but even in situations where their geological age cannot be as- 



