Vol. 69.] IN THE MAGNESIAN LIMESTONE AT HARTLEPOOL. 199 



V. Nature of the Shell-Limestone and the Associated Strata. 



One of the most conspicuous features of the Permian of Durham 

 is the ridge or bank of more or less regular, dome-shaped masses 

 of Shell-Limestone which lie in a belt from 2 to -i miles wide 

 in a north-westerly and south-easterly direction. Commencing 

 with a fragment on Tynemouth Cliff, it occurs north of the Wear 

 at Down Hill and Bylton Castle, passes by Claxheugh Bock and 

 Pallion, Humbledon and Tunstal] Hills, llyhope, Dalton-le-Dale, 

 Hawthorn, Beacon Hill, Easington Colliery, Horden, and Castle- 

 Eden Dene, until it touches the coast at Blackball Pocks ; after 

 this it reappears in Hesleden Dene, and thence presumably passes 

 under Hartlepool. 



The presence of these masses of Shell-Limestone and the condition 

 of the beds upon their eastern flank have been the determining 

 factor in the nature of the coast-section between Sunderland and 

 Hartlepool. 



The Shell-Limestones of the Middle Maguesian Limestone form 

 an irregular chain of reef-knolls, lying in a band more or less 

 parallel with the old Pennine shore-line. They are the chief 

 repository of the fossils of the Magnesian Limestone, the classical 

 localities being Humbledon and Tunstall Hills, both south of 

 Sunderland. 



The most probable view of the origin of these bryozoonal reefs 

 would seem to be that they represent areas of comparatively 

 sulphate-free deposition in the Permian sea, where the fauna, in 

 consequence of greater purity of the water, would congregate and 

 thrive. Assuming this, then, to be the true explanation, it will be 

 seen that this rock, owing to its greater freedom from more soluble 

 constituents, is to-day practically unaltered in volume ; while the 

 surrounding strata, more particularly the equivalents of the Shell- 

 Limestone upon its eastern flank, have undergone structural col- 

 lapse and degradation, leaving the Shell-Limestone as the elevated 

 ridge or bank which forms so conspicuous a feature of the geology 

 of the county. 



This supposition, the plausibility of which can hardly be denied, 

 in view of the already demonstrated replacement of Magnesian 

 Limestone by sulphates in the Hartlepool area, certainly affords a 

 ready explanation of many extremely puzzling features observable 

 at various points on the coast-line where beds of Upper Limestone 

 are brought down against, and frequently occur on a lower level 

 than, the bank of Shell-Limestone. 



The strata on the western and eastern sides of the ridge of Shell- 

 Limestone in the southern area of the county, that is, on the side 

 facing the Pennine shore-line and on the off-shore side of the Shell- 

 Limestone reefs, are markedly different one from the other. 



In the northern area of the county the western escarpment of 

 the Shell-Limestone occurs directly upon the Lower Limestone, 

 and generally only a short distance to the east of the great main 

 Permian escarpment. 



Q. J. G. S. No. 274. p 



