Mr. F. FoRSTJEn's Observations on the South Welsh Coal Basin. 87 



accompanying these observations was taken. The south edge of the Coal 

 Measures now range across the promontory of Gower until they are again 

 encroached upon by the sea in the form of a crescent, forming the fine 

 Bay of Swansea. About half wa}'' across this promontory, the Old Red 

 Sandstone becomes covered and concealed by the overlying bed of 

 Carboniferous Limestone, changing its dip to the southward.* Up to 

 this point, about 47 miles from the western end of the Coal-field, the 

 Carboniferous Limestone and Old Red Sandstone, forming the basis on 

 which the Coal Measures repose, basset, or crop out, along both edges of 

 the Basin, except in a portion of Pembrokeshire, extending to the 

 north-west of Milford Haven, where they rest on Trap and Greywacke 

 for about 10 miles on their south basset, and on Gi'eywacke for about 5 

 miles on their north basset. (^See the Plan and Sections by Mr. De la 

 Beche already referred to.) After an interval of 12 miles, occupied by 

 Swansea Bay, the southern edges of the Coal Measures again enter the 

 Land near Margam, and their total width from north to south has in- 

 creased to 20 miles, about the greatest width they attain. At this point, 

 th^ highly inclined basset edges of the Limestone and Old Red Sand^ 

 stone become partially covered over and concealed, by an overlying 

 and nearly horizontal deposit of Magnesian Limestone and Lias ; 

 this continues for about nineteen miles to the eastward, where 

 the Limestone and Old Red Sandstone finally emerge to the day 

 from under the newer formations, near the village of Pentirch, and 

 continue to maintain an elevation round the eastern termination of the 

 Basin, which the newer Rocks never reach ; near Pontypool the Basin 

 terminates in a semi-circular sweep, extending from north to south ; its 

 width at its termination is about 12 miles. The total length from 

 Pontypool to St. Bride's Bay, is 93 miles, and the area within the Lime- 

 stone, exclusive of the portions cut off by Swansea and Carmarthen 

 Bays, or in other words, the available portion of this Coal district is 



■ * This change in the dip of the Limestone, from north to south, prevails to such an 

 extent along the south edge of the Coal-field, that the existence of another trough of Coal 

 Measures, in the Bristol Channel, is by no means improbable. 



