Reports and Proceedings — Geological Society of London. 281 



Bedded yellow, non- 

 fossiliferous limestones 

 of the northern end 

 of Marsden Bay and 

 the coast from Hendon 

 to Seaham Harbour. 

 Often highly brecciated. 

 150 feet. 



(b) Middle. 

 (1) Unbedded (as a rule), highly 



fossiliferous (often) lime- 

 stone of Claxheugh, Tun- 

 stall, etc. Forms a ridge replaced 

 of high ground and reaches \ on the 

 a thickness of 300 feet. east by 



Often brecciated and en- 

 tirely changed in character 

 — rendered more calcareous 

 and fossils obliterated. J 



(c) 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 

 reassorted 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 percentage 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 palseonto- 

 logical features are the profusion of individuals in the Middle 

 Fossiliferous Limestone (which appears to have formed a shell-bank 

 in the Middle Magnesian Limestone sea), and their sudden dis- 

 appearance in the Upper Limestone. No Corals, Echinoderms, Polyzoa, 

 Brachiopods, or Cephalopods have ever been found above the top of 

 the Middle Fossiliferous division, only a few fishes, Gasteropods, 

 Lamellibranchs, Entomostraca, and Eoraminifera occurring in the Upper 

 beds. The Lower and Middle Fossiliferous Limestones are marked 

 by the presence of Productus horridus, Sow. Fish-remains occur at 

 two horizons : namely, the Marl Slate and the Flexible Limestone, 

 and the beds above these deposits. 



The Brecciated Beds, which occur at various horizons, chiefly, 

 however, in the two Middle divisions, constitute the most marked 

 tectonic feature of the Magnesian Limestone of the area. They have 

 been produced by thrusting, which brought about a decrease in the 



