496 



Mr. Logan on the Underclay 



presents a uniform parallelism in the faces of its cleavage, while the cleavage of the 

 older coal is parallel with the sides of the pebbles, which lie in all positions, and 

 very often exhibits the form of a rhomboid with its edges and corners rounded by 

 attrition. The coal of the pebbles, like that of the regular seams in the neighbour- 

 hood, is of the common bituminous kind, but among them have been found two 

 small ones consisting of cannel coal. 



The Cilfay sandstones, and the measures at Penclawdd, in which the first-men- 

 tioned pebbles were found, form part of the Pennant grit of Mr. Conybeare, and there 

 is reason to believe that throughout the whole of this great mass of sandstone, in 

 thickness about 3000 feet, there are occasionally beds of coal pebbles. 



I have not seen any of them associated with the lower seams. If none exist, it 

 might be inferred that the pebbles in the Pennant grit were derived from them, 

 after the coal-beds of the lower measures had been indurated, converted, and 

 crystallized. Now as the only beds of cannel coal in the lower measures, to yield 

 the pebble found at Penclawdd, are two or three lying about 2000 feet below the 

 coal-seam, it follows that the deposit of these 2000 feet must have required at least 

 the time necessary to convert vegetable matter into good pit-coal ; and if it is not 

 assuming too much to suppose that the deposit proceeded at an equal rate from the 

 beginning to the end, six times that period must have elapsed before the accumu- 

 lation of the whole coal-field of South Wales was completed. 



APPENDIX. 



Section of Coal-measures at Pemhray, in Caermarthensliire, South Wales. 



Ft. 



Sandstone or grit, hard and micaceous . 45 



Arenaceous shale 30 



Sandstone as before 32 



Arenaceous shale 1 



Sandstone 1 



Arenaceous shale 2 



Sandstone 39 



Argillaceous shale with a continuous band 



of ironstone 1 inch thick 3 



Sandstone -IS 



Argillaceous shale with a continuous band 



of ironstone 2 inches thick .... 2 

 Coal and carbonaceous shale in thin alter- 

 nate layers 4 



Underclay filled with Stigmaria Ficoides 2 



in. 

 

 

 

 6 

 

 6 

 





 



Arenaceous shale 



Coal 



Underclay filled with Stigmaria Ficoides 

 Argillaceous shale with a slight quantity 



of sand in it 11 



Coal and carbonaceous shale in alternate 



layers 



Underclay with Stigmariae 



Argillaceous shale 14 



Coal 



Underclay with Stigmariae 



Argillaceous shale 



Coal, bituminous. [This and the set of 



seams near it are called the Gwdig 



seams.] 10 



Ft. 



in. 







fi 







1 



2 



6 



11 



6 



1 







1 



6 



14 



6 







5 



1 







7 







