Murchison's Silurian System. 533 



the relative position of the masses on the map. These ap- 

 pearances can, however, be more simply explained by the 

 upward and downward movement of the faults in question, 

 and by the different angles at which the separate masses are 

 inclined. 



The limestone emerging from this morass at Paut-y- 

 gwasted, where the strike is true south-westerly, is well ex- 

 hibited in low parallel ridges crowned by high ridges of mill- 

 stone grit. The strata dip SE. and SSE. at angles of 

 about 30°. These calcareous terraces diminish successively in 

 height, and terminate near the cavern from which the river 

 Lwehwr bursts forth. Between this point and the house of 

 Cwrt-a-bard'dh the limestone is wanting, and the distance is 

 occupied by the old red sandstone or its detritus. 



The surface of the ground near the source of the Swehwr 

 is singularly marked by several funnel-shaped cavities, which 

 are not peculiar to the limestone of this spot, but are also 

 observable along the lines of greatest dislocation around 

 the promontory of Carreg-o-gof, and beneath the northern 

 face of Carreg-las. They seldom exceed 60 to 70 feet in 

 their diameter at the surface, tapering downwards to depths 

 of 30 to 40 feet, and apparently terminating in vertical tubes. 

 As they occur at or near points of the greatest dislocation 

 of the strata, may we not be allowed to speculate upon their 

 having formerly been the spiracles by which certain gases 

 were evolved, during those periods when earthquakes 

 produced the adjoining elevations and depressions of the 

 strata ? 



I invite special attention to the locality of Cwrt-a bard'dh, 

 because this portion of the margin of the coal basin is 

 directly opposite to the remarkable outline of carboniferous 

 limestone called Castell-cerrig-ceunen, with the position of 

 which I shall now attempt to prove that the break in the 

 escarpment is directly connected. 



The picturesque limestone rock on which Castle-ceunen is 



