286 Messrs. Buckland's and Conybeare's Observations on the 



ately connected with it, on the road between Newport and Chepstow, near 

 Penhow -J are two very considerable and boldly pointed mountains of this rock. 

 The elevations attained by the summits of the mountains of sandstone in this 

 neighbourhood, vary from 800 to 1300 feet above the sea. 



4. Mountain limestone. 



This important and extensive series of calcareous beds, is distinguished 

 from transition limestone, rather by its position, than by any very wide dif- 

 ference in its general character, or organic remains. 



According to the measurements of Mr. Mushet, to whose section we refer 

 for an account of the beds belonging to this formation in the Forest of Dean, 

 the total thickness of the mountain limestone is about 120 fathoms. The 

 mountain limestone of this basin agrees in all its general characters with that 

 of Somersetshire and Gloucestershire ; we may notice however, as peculiar 

 to it in the Forest of Dean, certain subordinate beds, which are so charged 

 with oxide of iron, as to have been worked as an iron-ore for many cen- 

 turies. 



The zone of limestone, belonging to this basin, is from a furlong to a mile 

 in breadth, according as the dip of the strata is more or less rapid. The an- 

 gle of dip on the northern and western border is often only 10°, but on the 

 eastern it frequently amounts to 80*^. The calcareous zone suffers only one 

 short interruption, scarcely 3 miles in length, on the south-eastern border, 

 near Lydney, where in consequence of a fault the limestone disappears, and 

 the coal-measures are seen in contact with the old red sandstone *. The line 

 of this fault is indicated by a series of springs, which break out at the ponds 

 in the Lydney valley, exactly between some coal-pits and a large flag-stone 

 quarry, worked in the old red sandstone. From Coleford, on the south- 

 western border of the zone, a branch of limestone, lying in a trough of old 

 red sandstone, proceeds southwards to the neighbourhood of Tinterne. This 

 calcareous branch contains a small insulated coal-basin not exceeding a mile 

 in length, situated half-way between Tinterne and the village of Wollaston. 

 Below Tinterne, the limestone forms the romantic cliffs which skirt the defile 

 of the Wye on either side, as far as the confluence of that river with the Severn. 

 Near Chepstow, are contortions in this limestone, which have been repre- 

 sented by Dr. Macculloch, in a drawing presented to the Geological Society. 

 The limestone on the right bank of the Wye, extends as far as the Crag 

 whereon Penhow Castle stands. It is bounded on the south by the marshes 



* A fault, suddenly bringing the same two formations into contact, has already been noticed 

 at Clapton on the northern foot of Leigh Down; and another, producing similar effects, is found 

 at Stanley, on the right bank of the Severn, between Bridgenorth and Bewdley. 



