CSO Geological Society. 



(4) Upper red beds with salt and thin fossil i ferous Mag- 

 ncsian Limestoues (only exposed iu the south of Durham). 

 300 ieet. 



(3) The Magnesian Limestone, 

 (rt) Upper. 



1. Yellow bedded limestone of Eoker. 100 feet. 



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



concretionary and non-concretionary limestones and marls. 150 to 

 2.')0 feet. 



3. The Flexible Limestone. 10 to 12 feet. 



(i) Middle. 



(1) Unbedded (as a rule), highly "» 



fossiliferous (often) lime- ( Bedded yellow, non - 



stone of Claxheugh, Tun- | ill fossiliferous limestones 



stall, etc. Forma a ridge I '' °^ *^'^ northern end 



of high ground and reaches [ ,,' J of Marsden Bay and 



a thickness of 300 feet, j ^t | '^''^ coast from Hendon 



Often brecciated and en- | r to Seaham Harbour, 



tirely clianged in character I ^ '■ Often highly brecciated. 



— rendered more calcareous ] ^_ 150 feet, 

 and fossils obliterated. ) 



(r) Lower. Bedded brown limestones of Frenchman's Bay, Houghton, 

 etc. Upper beds often disturbed. 40 to 200 feet. 



(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 syncline beneath Sunderland. 



The unfossiliferous Yellow Sands are probably a deltaic formation 

 reasserted by wind, the other beds being the result of de]iosition iu 

 an inland sea undergoing desiccation. The magnesium carbonate 

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

 ■with the calciiim 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 Icaching-out of the magnesium carbonate (dedolomitization), 

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

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

 fossils have been obliterated. They do not aflbrd 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 distributed. The marked 

 pahiiontological features are the profusion of individuals iu the 

 Middle Fossiliferous Limestone (which appears to have formed 



