164 DISLOCATIONS AND OUTLIERS OF CARBONIFEROUS LIMESTONE. 



the enormous masses of millstone grit and carboniferous limestone which formerly con- 

 nected " Pen Cerrig Calch " with the South Welsh coal-field, and the formation of the 

 present valley of the Usk, which is here five miles broad and nearly 2000 feet deep, 

 may both be ascribed to violent dislocations and the subsequent action of powerful 

 currents of water ; this valley is, therefore, unquestionably " a valley of denudation." 

 (See PL 31. fig. 1.) 



The term denudation, however, has been made use of to express so many operations 

 of water, differing from each other in intensity and in their nature, — some geologists 

 meaning to express thereby, sudden ravages occasioned by a transient but powerful 

 flood, others seeing in it nothing more than the gradual work of rivers, formerly in a 

 more violent state of action, — that it is well to explain what may have been the opera- 

 tions employed in producing the phenomena in question. The longitudinal depres- 

 sion in which the Usk now flows, is simply a great line of fault, determined no doubt by 

 one of those movements of elevation which have broken up the escarpment of the coal 

 basin and thrown it into its present form. Earthquakes and subterranean forces, suf- 

 ficient to upheave the stupendous mountain masses of conglomerate and sandstone of 

 the Fans and of the Black Forest, and to snap off huge portions of the chain, as well 

 as raise them into the circular escarpments of the coal-field, could not have acted 

 on such large accumulations of solid sedimentary matter, without occasioning immense 

 transverse openings or gullies. On the supposition that these oscillations were going 

 on for a long time before the deposits were raised above the surface of the waters, how 

 powerful must have been the submarine currents set in action by these changes of 

 level ! How must they have affected the bottom of the sea ! How deeply must such 

 currents have channelled out the hollows into which they were deflected ! By all these 

 combining causes it is conceived, that the valley in question was both determined and 

 subsequently deepened and increased. 



Whatever hypothesis may be advanced to account for the present position of the 

 lofty outlier of Pen Cerrig Calch, the detritus of those materials, which formerly 

 united it with the South Welsh coal basin, is still piled up in vast mounds and terraces 

 of gravel on both banks of the Usk to attest the work of destruction, the effects of which 

 will be further noticed in the two concluding chapters of this work. 



2. Other dislocations and outliers of the Carboniferous Limestone. 



Passing unnoticed all minor derangements, I shall here describe great dislocations 

 only, which have affected the carboniferous limestone and upper portions of the Old 

 Red System, and have tended to give the South Welsh coal-field its present configu- 

 ration. 



The tract in which they occur is included between the Caermarthenshire Fan on the 



