490 Dr. D. Woolacott — Magnesian Limestone of Durham. 



It probably, however, marks a decrease in tbe depth of the Permian 

 sea on the east side of the reef, due to infilling by sediment and 

 desiccation of the area. The Upper Limestones, which thicken 

 eastwards, were thus laid down in shallower water and over a, more 

 restricted westward area than those below. They are thus retro- 

 gressive with respect to both the Lower and Middle Divisions. Their 

 lower beds are equivalent to the higher parts of the reef. This can be 

 proved by a study of the sections along the coast, and from the fact 

 that in Blackhall Colliery sinking the Concretionary Limestone 

 rested directly on the reef. 1 They lie in a broad syncline stretching 

 from Marsden to the south of Sunderland, and are faulted-in at 

 Seaham Harbour. Along the coast to the south one or two small 

 synclines and faulted-in areas occur, and at Hartlepool they are 

 well exposed. They consist of the highly altered concretionary 

 limestones and of the Poker and Hartlepool oolites. The latter 

 consist mainly of strata composed of minute hollow spheres, which 

 were originally gypsiferous oolites. 2 These oolitic beds were probably 

 not formed in water of any great depth, as King records ripple- 

 marked limestones from their upper layers. 



Above the Upper Limestones occur red marls, false-bedded sand- 

 stones, thin fossilif'erous dolomitic limestones with salt, gypsum, 

 and anhydrite. They occur to the west of the reef, but are of 

 limited westward extension, which was pointed out by Lebour 

 some years ago. They were also deposited to the east of their 

 present Permian outcrop, as Trechmann has proved that similar beds 

 were carried in by the Scandinavian ice-sheet and left in the 

 glacial deposits near Castle Eden Dene. 3 They mark the drying up 

 of the Permian sea, the final stages of which are represented by the 

 deposits of the more soluble salts of calcium, magnesium, sodium, 

 potassium of the Stassfurt area, which were probably deposited far 

 to the west of their present proved position. 



The Permian of Durham can be readily correlated with that of 

 Thuringia. 4 The similarity of the Magnesian Limestone with the 

 Zechstein proves the conditions of deposition were the same. In 

 both cases they were laid down on the edge of the Permian sea, 

 which must have stretched during a contracted period continuously 

 between the two areas. 



In the examination of any subsequent borings that may be made 

 in South Durham or Yorkshire there are several points of interest 



1 Trechmann, Q.J.G.S., vol. Ixix, p. 213, 1913. 



2 The hollow spherules in parts of this rock are very irregular, and have been 

 referred to as being pseudo-oolitic and concretionary. The study of the oolites 

 in other parts of the limestone and their microscopic structure prove that both 

 the regular and irregular spherules are ooliths. They are inorganically-formed 

 oolitic limestones formed by the deposition of dolomite round gypsum. When 

 this substance dissolved out the dolomitic spherules was also partly removed. 

 The ooliths with the gypsum still remaining in them from the cores of borings 

 in South Durham show perfect oolitic structure. 



3 Q.J.G.S., vol. lxxi, p. 65. 



4 See table giving this correlation in my paper on Permian of North Durham, 

 op. jam cit., p. 254. 



