532 Murchison's Silurian System. 



the old red sandstone of the Ferns is proved to have been 

 upcast to an extent of at least 700 feet. The direction of 

 this transverse fault is nearly from north to south, as mark- 

 ed by the fissure in which the rivulet Twrch-fechan flows 

 in its descent from the edge of the escarpment toward the 

 centre of the coal basin. On the left bank of this rivulet 

 all is old red sandstone, while a little to the right is the 

 millstone grit of Carreg-las, the limestone being entirely lost 

 for the space of nearly two miles. The elevated mass of 

 old red sandstone dips ten to twelve degrees SSE., the 

 limestone and millstone grit being inclined twenty degrees 

 to the south. This inclination of the limestone accounts for 

 its rapid disappearance beneath the millstone grit of Carreg- 

 las, and connected with the great upcast of the Ferns, ex- 

 plains how the latter rock has been thrown into juxtaposition 

 with the old red sandstone. 



Immediately to the west of the great limeworks of Clogan- 

 maur, there is another considerable fault. After occupying 

 the terrace-formed ridge, on which the kilns are situated, the 

 limestone is abruptly snapped off, and thrown down about 

 200 feet beneath its usual level into a morass, the ends of the 

 lower strata only being visible on the side of the mountain 

 rill. This is the spot above Blaeu-Cennen, where the lime- 

 stone presents the peculiar glazed surface before described. 

 The strike of this dislocated mass is 10° to the north of 

 west, and the inclination, differing from that of the adjoining 

 lime quarries, is 25° south by west. This downcast, though 

 not exhibiting so great an amount of disturbance as that of 

 the upcast of the old red sandstone in Fan-sirgaror, exhibits 

 in one respect analogous phsenomena. In both cases large 

 portions of the lip of the coal-field present the appearance 

 of having been extruded from their regular line of bearing, 

 and forced into the area of the old red sandstone by move- 

 ments which miners would call " lateral shifts ;" an expla- 

 nation which would naturally suggest itself by inspection of 



