Dr. D. Woolacott — Magnesian Limestone of Durham. 487 



distinctly traceable, running across country for a distance of 

 20 miles. It is dissected on Down Hill (Boldon Hills) and 

 Claxheugh, near Sunderland, and to the south can be seen on the 

 knolls and old quarries of Humbledon and Tun stall Hills, and in 

 Fox Cover Quarry, Seaham Harbour. One of the best fossiliferous 

 exposures occurs on the flanks of Beacon Hill, and it is exposed in 

 old quarries at Easington and Horden, finally disappearing beneath 

 the upper beds south of Castle Eden Dene. The sinking at Blackball 

 Colliery went right through this reef on its eastern flank, and 

 Trechmann was able to study its fauna in an exceptional way. It 

 was there 335 feet thick and was overlaid by Concretionary Lime- 

 stone. In appearance and fauna it is similar to the Bryozoa Reef of 

 the Zechstein of Thuringia. 1 Trechmann considers it to have been 

 formed 10 or 12 miles from the western shore of the Permian sea. 2 

 It was originally a shell-bank — the Brachiopods and other forms 

 sheltering among the Bryozoa — upon which was settling fine 

 calcareous and dolomitic sediment. A great portion of the organically 

 formed part of the limestone has undergone secondary dolomitization, 

 either pene-contemporaneously with deposition or long after (both 

 processes may have taken place), but some parts of the whole lime- 

 stone ai - e still calcareous. Near the reef in the off-shore equivalents 

 there is an interbedded fossiliferous breccia, which was a Yor-reef 

 formed of blocks broken off by the waves, 3 i.e. the Shell-Limestone 

 Conglomerate of Howse exposed at Blackhall Rocks and in Hesleden 

 Dene 4 (Photograph, PL XII, Fig. 2). On its western side current- 

 bedded oolitic dolomites have been noticed in Silksworth Quarry, 

 and some of the fossils in parts of the western equivalent of the 

 reef appear to have been drifted from it. The fauna of the Permian 

 sea of Durham received a sudden influx of species along the area 

 now occupied by this shell-bank owing to the depth, temperature, 

 and salinity of the water offering fairly congenial environment. 

 About thirty invertebrate species lived in the Lower Limestone 

 sea in small numbers, but in the reef a hundred flourished in the 

 greatest profusion under, however, rather hard conditions, as they 

 never reached any large size and gradually underwent extinction, 

 only about sixteen species being left to continue a protracted 

 existence during the deposition of the Upper Limestones, finally 

 disappearing from the area when the Upper Red Beds with salt 

 were being deposited. Brachiopods, polyzoa, corals, echinoids, 

 and cephalopods all died out with the passing away of the reef 

 conditions, none of these ever being found in the Upper Lime- 

 stones. On its east or off-shore side the reef is replaced by yellow, 

 bedded, unfossiliferous dolomites and segregated limestones, which 



1 E. Kuste, Geologisches Wanderbucli fur Ostthilringen imd Westsachsen, 

 p. 141. J. G. Bornemann, "Von Eisenach nach Thai und Wuther " : 

 Jahrb. konigl. preuss. geol. Landesanstalt, 1883. In 1913 Trechmann and 

 I visited Thuringia in order to compare the Permian rocks of Durham and 

 South Germany. 



2 Op. jam cit., vol. lxx, p. £59. 



6 A Vor-reef also occurs in Thuringia (E. Kuste, op. jam cit., p. 141). 

 4 The coarse fragments in this bed are more or less rounded. 



