COAL MEASURES. I93 



The general thickness in the Denbighshire Coal-field is as 

 follows : — 



Upper Series. — Red and grey sandstones, red clays, shales, and thin coals, 



with Spirorbis Limestone. looo feet. 

 Middle Series. — Sandstones and shales, with coals and ironstone. Fish remains, 



Anthracosia. 800 feet. 

 Loioer Series. — Grits, flags, and shales, with thin coals. Goniatites. lOOO feet. 



In North Flintshire the Millstone Grit is overlaid by the Gwespyr 

 Sandstone (upwards of 100 feet thick), and by the Holywell Shales, 

 which occur near Holywell.^ The Shales contain bands of lime- 

 stone which have yielded Posidotiomya Gibsoni, Avicidopecten papy- 

 raceus, etc. 



Near Leeswood, south-east of Mold, there is evidence of con- 

 temporaneous erosion in the Coal-measures, as Mr. A. Strahan 

 informs me that a channel filled with debris of Cannel Coal, 

 etc., occurs in the Coal-measures. This is a feature like the 'horse' 

 in the Forest of Dean. 



In the Denbighshire Coal-field the Coed-yr-allt rock, 30 to 60 feet in thickness, 

 a greenish-white and grey sandstone, belonging to the Upper Coal-measures, has 

 been quarried. The Newbridge shales belong to the Upper Coal-measures in 

 this district. These rocks have been described by Mr. D. C. Davies, who has 

 drawn attention to the occurrence of coal-seams in beds which he regarded 

 to be of Permian age near Ifton, between Wrexham and Oswestry in Shropshire. - 

 There is little doubt, however, that these beds are iron-stained Coal-measures. 

 At Oswestry there is but one bed of good coal. 



A small patch of red unproductive Coal-measures rests on the 

 Carboniferous Limestone opposite Caernarvon. A band of un- 

 productive Coal-measures forms a narrow strip of coast-line be- 

 tween Caernarvon and Llanfair-is-gaer, and is succeeded by a belt 

 of Carboniferous Limestone thence to Gored-girth on the coast 

 near Bangor. 



In Anglesey a little tract of Coal-measures is preserved at Mall- 

 draeth Marsh, owing to a great fault, which has a downthrow of 

 upwards of 2000 feet. The Coal-measures attain a thickness of 

 over 1300 feet, and contain seams from one to nine feet in 

 thickness, which have been worked.^ 



14. SOUTH WALES COAL-FIELD. 



This is the largest Coal-field in England and Wales. It occupies 

 parts of the counties of Monmouth, Glamorgan, Caermarthen, and 

 Pembroke, including an area of about 900 square miles. The 

 Coal-measures are developed to the south of Haverfordwest, and 

 again at Swansea, Neath, Llantrissant, Caerphilly, in the Taff and 

 Rhondda Vales, at Pontypool, Tredegar, and Merthyr Tydfil. 



^ A. Strahan, Geology of Rhyl, etc. (Geol. Survey), p. 20. 

 ^ Proc. G. Assoc, iii. 138; Q. J. xxxiii. 10, xli. 107 (Proc). 

 ^ A. C. Ramsay, Geology of North Wales (Geol. Survey), edit. 2, pp. 10, 

 II, 261. 



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