internal Structure of the Magnesian Limestone. 49 



narrow denudation, which has removed all the limestone escarpment south of Thrisllngton • with 

 the exception also of a second denudation, which affords a passage for a rail. road, and has swept 

 away all the limestone from West llerrington to East Herrington. 



Neither of these two remarkable gaps in the formation is represented in the geological map 

 of Durham. The extension of the limestone below the escarpment at Hetton.le-Hole is also 

 omitted ; and there are some other slight inaccuracies in the outline necessarily originating in the 

 false engraving of the map. 



The demarcation sweeps round the summit of Painshaw Hill, and there turns parallel to the 

 river along the top of the escarpment, passes just to the north of Offerton, descends in an undu. 

 lating line to the north of High Ford, down to Clacks Heugh, and thence by Low Ford to the 

 bank of the Wear above Pallion. The passage of the formation across the river is disguised; but 

 the limestone gradually ascends on the other side to the north-west, and passes close to Hilton 

 Castle, which is at the bottom of an obscure escarpment. From Hilton Castle the line ranges 

 for some way immediately on the north side of the road, then deviates to the north-east, and 

 afterwards sweeps round the front of West Bolden Hills, and passes into the lower part of the 

 village, from which place it ranges immediately on the north side of the road to East Bolden. A 

 little to the north-east of the last-mentioned place the boundary is lost in some marshes. It ap- 

 pears, however, to pass near Tile-Sheds ; and from that place it ranges a little way to the west 

 of Cleadon, Harton, and Westoe. It is seen on the east side of the road from Westoe to South 

 Shields ; but it does not extend to the town, and its further range to the north-east is disguised 

 by the hills of blown sand, which extend along the coast about two miles from the mouth of the 

 Tyne*. 



The magnesian limestone, as is well known, forms the capping of Tynemouth Castle Hill. For 

 about two miles north of the castle hill, the cliff is composed of coal strata ; but in the small head- 

 land immediately south of Cullercoats, the limestone again appears. This is the most northern 

 point at which the formation is found on the coast. It is, however, seen in the neighbouring 

 quarries near Whitley, of which a detailed account is given by Mr. Winch. (Geol. Trans, vol. iv. 

 p. 4.) The masses of limestone at the three last-mentioned places are, I believe, unconnected 

 with each other, and might without impropriety be considered as outliers f. 



Such are the results of a series of observations made for the purpose of determining the limits of 

 the magnesian limestone. The details are necessarily tedious, and may perhaps appear trifling. 

 As however they are the result of considerable labour, and are absolutely necessary to a more 

 correct delineation of the formation, I hope they may be considered of sufficient importance to be 

 recorded. 



Eastern boundary/ of the Limestone. 



It appears from the details given above^ that the escarpment of the mag- 

 nesian hmestone may generally be laid down with precision ; and an exami- 

 nation of its range leads to the discovery of many objects of great interest. 



* From Painshaw Hill on the right bank of the Wear to Tynemouth, the boundary line is ex. 

 tended by Smith too far to the west. A part of this line is, however, difficult to determine ; for 

 all the higher portions of the formation are seen in a chain of low round-topped hills, which 

 stretches considerably to the east of the true boundary. 



t The representation of the magnesian limestone in the geological map of Northumberland is 

 erroneous. It is there made to extend uninterruptedly along three or four miles of the coast. 



V0# III. — SECOND SERIES. H 



