CARBONIFEROUS LIMESTONE OF LLANYMYNECH, ETC. 



145 



As the red variety of the millstone grit to the east of Sweeny is in juxtaposition 

 with the Lower New Red Sandstone, the exact line of demarcation between the two 

 formations becomes difficult, particularly where the surface of the lower grounds is 

 much encumbered by detritus. The apparent similarity of the red rock of the mill- 

 stone grit to that of the Lower New Red being likely to mislead speculators who do 

 not comprehend the structure of the district, I would therefore repeat, that the first- 

 mentioned rock distinctly underlies the coal measures, and reposes on the limestone, and 

 hence that any effort to seek for coal beneath it would be absurd. 



Carboniferous Limestone (/. of section). 



The limestone of this tract (generally known as the limestone of Llanymynech) is by 

 far the finest example of the formation in Shropshire, and extending on the slopes of 

 the higher hills into Denbighshire and Flintshire, it may be well entitled to the old name 

 of "mountain limestone." It is to be observed, however, that even in this range, the 

 limestone is never of such dimensions as to form of itself a mountain mass, for where 

 it attains the height of fourteen and fifteen hundred feet above the sea, it is nothing 

 more than a band, resting upon the older Silurian rocks, which in reality form the chief 

 body of the mountains. (See Sect. PI. 30. fig. 14.) At a moderate computation, in- 

 cluding the impure strata, it has however a maximum thickness of at least four or five 

 hundred feet. It is thrown into ridges rising from beneath each other, and broken 

 through by many transverse fissures, some of which amount to valley gorges. At 

 Llanymynech the limestone presents a bluff escarpment resting on highly inclined 

 edges of Silurian schists, and dipping away at moderate angles. After ranging from 

 Llanymynech to the north-north-east, it is deflected to the north-west, passing by 

 Porth-y-wain to Treflach wood and Trefonen 1 . Thence it is thrown still further west- 

 wards by the Carneddew, from which place to Pen Coed y Gaer, it is flanked by a 

 ridge consisting of porphyritic trap rock, and schists and sandstones of the Silurian 

 System. At the point called Sir Watkin's Tower, 1200 feet high, the limestone appears 

 within one hundred yards of the trap rock of Pen Coed y Gaer. It thence trends to 

 the north-east in a bold rocky escarpment called the Craig y Rhiw, passing at Llawn 

 the road from Llanarmmon to Oswestry. It is then lost for a short space, but again 



south of Mold, contains strata of the intermediate characters described near Oswestry, some of " the bastard 

 limestones having quartz pebbles bedded in a calcareous cement." In the Bristol district strata of this age are 

 classed as upper limestone shale by Buckland and Conybeare. 



1 The Ordnance map of these parts not being published when my last visit took place (1836), I could not ac- 

 curately define these breaks, and if my readers find this portion of the accompanying map the least accurate in 

 the delineation of the outline, they must attribute it to the want of that invaluable assistant to the field geolo- 

 gist, an accurate survey, without which indeed the map accompanying this work could never have been under- 

 taken. The trap rocks are indicated on the map, but I shall not particularly describe them on this occasion, as 

 I could not discover them in absolute contact with any. member of the Carboniferous System. 



