6S0 



Geological Society. 



replaced 

 on 

 the 

 east 



by 



( Beclcled yellow, non - 

 ! fossiliferous limestones 

 I of the northern eud 

 of Marsden Bay and 

 the coast from Hendon 

 to Sealiam Harbour. 

 Often highly brecciated. 

 150 feet. 



enchman's Bay, Houghton, 

 200 feet. 



(4) Upper red beds with salt and thin fossiliferous Mag- 

 nesian Limestones (only exposed in the south of Durham). 

 300 feet. 



(.3) The Magnesian Limestone. 

 («) Upper. 



1. Yellow bedded limestone of Roker. 100 feet. 



2. The concretiouiry limestone of Fulwell and Marsden — a series of 



concretionary and non-concretionary limestones and marls. 150 to 

 250 feet. 



3. The Flexible Limestone. 10 to 12 feet. 



(h) Middle. 



(1) Unbedded (as a rule), highly "^ 

 fossiliferous (often) lime- 

 stone of Claxheugh, Tun- | 

 stall, etc. Forma a ridge ! 

 of high ground and reaches ! 

 a thickness of 300 feet. \ 

 Often brecciated and en- | 

 tirely changed in character 

 — rendered more calcareous | 

 and fossils obliterated. ) 



(c) Lower. Bedded brown limestones of Fi 

 etc. Upper beds often disturbed. 40 to 



(2) The Marl Slate. 3 feet. 



(1) The Yellow Sands, from to 150 feet. 



These beds, which vary much in thickness, lie in North Durham 

 in the general form of a sj'ncline beneath Sunderland. 



The unfossiliferous Yellow Sands are probabl)^ a deltaic formation 

 reasserted by wind, the other beds being the result of deposition in 

 an inland sea undergoing desiccation. The magnesium carbonate 

 existed in the waters of the sea, and was either deposited along 

 with the calcium carbonate, or introduced by seepage when the beds 

 were being laid down. 



Great changes in the amount and distribution of these carbonates 

 has, however, taken place since deposition. The cellular structures 

 that occur in the limestone can be classified as follows : — (1) Con- 

 cretionary-cellular ; (2) negative breccia; (3) solution - cavities ; 

 and (4) fractured cellular. Most of them have been produced by 

 the leaching-out of the magnesium carbonate (dedolomitization), 

 or of both that and calcium carbonate. In some cases the rock 

 has been rendered crystalline, as well as more calcareous, and the 

 fossils have been obliterated. They do not afford any proof that 

 the rock has been dolomitized subsequent to deposition. The per- 

 centage of calcium carbonate is sometimes over 99, while that of 

 magnesium carbonate is occasionally as much as 50. 



The fauna of the Magnesian Limestone is very restricted (about 

 140 species) and most peculiarly distrib;ited. The marked 

 palaeoutological features are the profusion of individuals in the 

 Middle Fossiliferous Limestone (which appears to have formed 



