160 ABSENCE OF COAL IN THE CARBONIFEROUS LIMESTONE OF SOUTH WALES. 



nations of thin-bedded, reddish and purple shale, yellowish sandy limestone and sandstone, and 

 finally by flaglike purple red sandstone, forming the upper part of the Old Red System. The 

 alternating beds of impure limestone, &c., as developed between White Mill and Rug's Hole, are 

 so unlike any other beds of the carboniferous limestone, that if they happened to be thrown up in 

 arched forms, (as is the case in adjacent districts 1 ,) they might at first sight puzzle any observers, 

 since they have much the character of the lower Silurian rocks, like which the sandstone beds are 

 also frequently marked by the frequent impression of crinoidal stems. In this section, however, 

 and again near Tintern Abbey and in parts of Pembrokeshire, the position of these beds is so clear, 

 i. e. they so distinctly overlie every portion of the Old Red Sandstone, and further, contain the car- 

 boniferous fossils, that they are completely established as the true beds of transition between the 

 carboniferous and Old Red Systems ; the precise boundary line being drawn at that part where traces 

 of the fossils and shale of the carboniferous system finally terminate, and the red sandstone fairly 

 sets in 2 . 



Natural sections such as we have been considering, and which lay open the transi- 

 tion of one group of rocks into another, are only to be sought in these regions where 

 the conterminous systems are much developed. This expansion of the carboniferous 

 limestone is so considerable in the eastern division of Monmouthshire and adjacent parts 

 of Gloucestershire, where the beds of passage are best seen, that the formation, which on 

 the edges of the South Welsh coal-field rarely if ever exceeds five hundred feet, has here 

 probably a maximum thickness of one thousand feet ; for in the Forest of Dean, where 

 the upper and lower shales are attenuated, as compared with the same beds between 

 Chepstow and Usk, Mr. Mushett, an eminent practical miner, has determined the dimen- 

 sions of the formation to exceed seven hundred feet. The passage from the lowest beds 

 of the limestone into the system of Old Red Sandstone, is analogous to that which I 

 have previously pointed out in the central counties of England, between the uppermost 

 part of the carboniferous strata, and the lowest part of the new red sandstone, the phe- 

 nomena in all these cases being displayed, in those districts where the respective portions 

 of these systems are the most developed. (See chapter 4.) 



I cannot terminate this account of the structure of the carboniferous limestone and 

 associated rocks, without reverting to a phenomenon to which I directed attention in 

 describing the Salopian coal-fields, and which is equally apparent throughout South 

 Wales ; viz. the entire absence of coal, either in the millstone grit, or underlying limestone ; 

 while in the North of England, on the contrary, the writings of Sedgwick. Phillips and 

 others, have taught us, that these lower members of the carboniferous system, are there 

 made up of almost indefinite alternations of sandstone, limestone, shale, coal, and iron- 

 stone. This neat separation of the carboniferous system of the central counties of 



1 At the Abbey one mile north of the Ship Inn near Alveston, and eleven miles north of Bristol, a small boss 

 of these beds has been recently cut through in lowering the road. In the fine section of the Avon these beds 

 also exist in full force. (See description and sections, hi a subsequent chapter on Tortworth.) 



2 Professor Phillips, in his second volume of the Geology of Yorkshire (p. 15 et seq.), has described alter- 

 nating red sandstone and limestone of this age, near Kirby Lonsdale. 



