18. 



Castle Cerrig Cennen. (From a drawing by Mrs. Stackhouse Acton.) 



CHAPTER XIII. 



CARBONIFEROUS SYSTEM {concluded). 



Principal Outliers and Dislocations of the Carboniferous Limestone on the northern 

 and western edges of the South Welsh Coal Basin. 



}. Pen Cerrig-calch. (PI. 31. fig. 1.)— THE chief masses of the carboniferous lime- 

 stone being described, it now only remains to give a sketch of the prominent disloca- 

 tions and outliers of the formation in South Wales. 



The strata of Old Red Sandstone, carboniferous limestone, and millstone grit, com- 

 posing the margin of the South Welsh coal-field, are all clearly exposed to the south 

 of Crickhowell, where the river Usk flows in a deep and broad valley between these 

 rocks and the loftier mountains of Old Red Sandstone constituting the Black Forest. 

 Observing in the Ordnance map that the summit of one of these mountains was marked 



