1850.] DAVIS ON THE NASH LIMESTONE. 433 



The first surmise I heard that this Umestone is referable to a lower 

 bed, was from Professor Sedgwick, who in the summer of 1846 took 

 a rapid glance at the district. Finding the limestone resting on beds 

 of undoubted Caradoc sandstone, and not at the time being aware or 

 recollecting that any other opinion was entertained, he inferred that 

 the formation was identical with that of Woolhope. More recently, 

 the officers of the Geological Survey of Great Britain have given un- 

 mistakeable indications of a similar opinion * . 



The true character of this limestone becomes a question of con- 

 siderable interest, as upon the accurate definition of the beds in Silu- 

 ria proper must in a great measure depend the correct knowledge of 

 their prototypes in other regions. 



To elucidate this question it is necessary to state, very briefly, the 

 geographical position and relation of the limestone to the adjacent 

 country. 



The sandstone beds above-mentioned occupy the summit ridge and 

 greater part of a hill south-west of Presteign called Cam or Caen 

 Wood, which rising to the height of about 1000 feet from the 

 sea-level, forms the northern boundary of the valley of Knill. The 

 strike of these beds of sandstone and grit (N.E.) is nearly identical 

 with that of the more extensively developed Caradoc sandstone in 

 Shropshire and Caermarthenshire, and with which they form an in- 

 termediate link, separated however by overlying deposits in both di- 

 rections. Between Caen Wood and the south-west point of the 

 Shropshire Caradoc beds, a distance of about twenty miles, the only 

 observed connection is a slight upcast of grits in the Park of Brampton 

 Brian f. 



The strata of sand and grit of Caen Wood form an anticlinal line 

 dipping N.N.W. on the north or Presteign side, at an angle of 50°, 

 and S.S.E. on the south flank of the hill, at a similar angle. The 

 northern dip is seen above the old and now abandoned excavations of 

 limestone at the * Sandbanks,' half a mile south of Presteign Gaol. 

 The southern or reverse dip is seen on the side of the turnpike-road 

 leading from Corton towards Nash, and in various parts of the wood 

 overhanging the same road. At the quarry near Corton turnpike- 

 gate the bedding is indistinct, and Sir Roderick Murchison observes, 

 that the rock, which is here a conglomerate, strongly resembles a vol- 

 canic grit ; and there can be no doubt that he is correct in assuming 

 the elevation of these beds to be due to the eruptive action at Old 

 Radnor, a few miles to the south-west, the force of which prolonged 

 to the north-east has resulted in the anticlinal hue of the Ludlow 

 Promontory and the valleys of elevation of Kinsham and Wigmore. 



The great bulk of the Nash limestone lies on the south side of the 



* See abstract of paper by Messrs. Ramsay and Aveline on the Structure of 

 parts of North and South Wales, Quart, Journ. Geol. Soc. vol. iv. p. 294. 



t This is mentioned in the paper already cited by Messrs. Ramsay and Avehne. 

 It v/as I believe first discovered in the autumn of 1838 by the Rev. T. T. Lewis, 

 who communicated the fact to Sir R. Murchison at that time, previous to the 

 publication of the * Silurian System,' but too late for its insertion ou the map. 



