Mr. F. Fobster's Observations on the South Welsh Coal Basin. 99 



will render the working of these seams, beyond a moderate depth, ex- 

 pensive, and in many cases impracticable. 



OBSERVATIONS ON THE STRATA, IN THE LINE OF THE 

 SECTION (sse Plate XIJ. 



The northern extremity of the section commences at the outbreak of 

 the Limestone, on Mynydd Mawr (or tlie Great Mountain), in Car- 

 marthenshire, at about two miles to the west of the village of Llandibie, 

 which is nearly half way between the western and eastern extremities 

 of the Basin ; from hence the line passes southward in the direction 

 of the dip of the strata, to the Llwchor River near Plas Llanedi, 

 about 31 miles above the bridge at Pont-ar-ddulais, where the mail 

 road to Milford crosses that river. This part of the section, 5 miles 

 700 yards in length, forming the 1st division, is constructed from a sur- 

 vey made in the year 1825. 



The Old Red Sandstone forming the basis on which the Limestone 

 reposes, is of very great thickness; its upper part is conformable to the 

 Limestone, but its lower beds pass by insensible degrees into Grey- 

 wacke Slate, at about 2 miles to the northward. The surface descends 

 rapidly in that direction under the bolder and more elevated escarpment 

 formed by the Limestone and Millstone Grit. 



The Carboniferous Limestone occurs in layers of from 2 to 7 f^^t in 

 thickness, between which, thin beds of Shale are occasionally interposed: 

 its aggregate thickness is 120 fathoms, and its dip or inclination in the 

 direction of S. IH" W. at an angle of 29°. Its general character is that 

 of a compact blue Limestone, but some of its lower beds pass into black 

 Marble abounding in Encrinites, and are quarried at various points 

 along their line of basset. Although this Limestone has been so ex- 

 tensively quarried, I am not aware that veins of Lead Ore of any 

 consequence have been found to intersect it. 



The Millstone Grit forms a line of barren hills immediately within 

 the outbreak of the Limestone, the rock frequently rising to the surface; 

 it is composed of a number of beds of Chert and Conglomerate, alternat- 

 ing with each other, and the former occasionally contains vegetable 

 organic remains ; its aggregate thickness is 85 fathoms. This forma^. 



