Yol. 69.] ANHYDRITE- MASS IN THE MAGNESIAN LIMESTONE. 201 



the eastern side ; but the Concretion ary Series and the overlying 

 Hartlepool or Eoker Beds are, of course, full in places of their 

 scanty but characteristic fauna. 



In my opinion 1 part of the Upper Limestones owe their present 

 position on the side of, and at a much lower level, than the 

 Shell-Limestone, to an irregular series of slip-faults caused by 

 the degradation of the underlying, originally sulphate-bearing 

 equivalents of the Middle Series. 



As a general rule, therefore, in the southern part of the county, 

 where the Shell-Limestone retreats from the shore-line or passes 

 beneath the surface, the Upper Limestones are exposed in the 

 shore-section. Where the Shell-Limestone approaches the shore or 

 forms a high elevation immediately behind the coast, the shore- 

 line and cliff-section are occupied by the highly-brecciated but 

 well-bedded Middle Limestones occupying the lower flanks of the 

 Shell-Limestone on its eastern side. These beds constitute large 

 stretches of the cliff-section to the north of the area at present 

 under discussion, and in general both these and the Upper Lime- 

 stones dip rapidly away eastwards from the Shell- Limestone. 



The extreme complexity of parts of the Durham coast-section 

 will, therefore, be readily realized. 



The typical Shell-Limestone is in general a bard, fine-grained 

 magnesian rock, usually without very definite planes of bedding 

 except in its upper parts, but is easily recognizable by its organic 

 remains. 



The accumulation of certain forms of marine life in the Shell- 

 Limestone may have been induced by the action of currents in the 

 Permian Sea, with consequent increase in the rapidity, and change 

 in the character, of the sedimentation. Whatever may have been 

 the inducing cause or causes, however, it is a true reef accumula- 

 tion, the reef-builders in this case being predominantly bryozoa 

 (taking the place of corals), brachiopods, lamellibrauchs, gasteropods, 

 ostracods, and, in the lower beds, crinoids. 



1 This is not mere conjecture. The faults which bring down the concre- 

 tionary beds against the Shell-Limestone north and south of Blackball Eocks 

 are of this order. 



At Beacon Hill, a knoll of Shell-Limestone facing the coast immediately 

 south of Hawthorn Dene, the brecciated beds now lie in a sort of platform at 

 the base of the hill, and form the whole of the cliff-section. A fissure in tbe 

 breccia contains fallen masses of Upper fetid and fossiliferous Limestones of 

 tbe concretionary series. The whole mass now occupies a lower level th%n the 

 Shell-Limestone, with a fauna placing it about the middle of the Shell-Lime- 

 stone and including Productus korridus. This rock is well exposed in a cutting 

 for the new Coast Bailway, which runs at the foot of the hill upon the platform 

 of breccia. Part of the side of the Shell-Limestone reef has been stripped off in 

 the railway-cutting, and slickensided surfaces are seen on the bank of Shell- 

 Limestone passing vertically down the slope, indicating the downward 

 movement of beds formerly adjacent to the now bared slope of Shell-Limestone. 

 Tbe Middle Breccias in the cliff are much segregated and unfossiliferous. 



Beacon Hill, therefore, represents the original eastern slope of the ancient 

 Permian reef, and is one of the best sections of Shell-Limestone at present 

 exposed. A quarry at the top of the hill shows a bedded Upper Shell- 

 Limestone with a typicallv impoverished fauna. 



p2 



