202 UK. C. T. TRECHMANN OX A MASS OF ANHYDRITE [June I913, 



There appears to have been a comparatively rapid though tranquil 

 accumulation of remains of these organisms. The delicate hollow 

 spines of Stroplialosia and Productus are preserved intact, and the 

 valves of lamellibranchs throughout the (Shell-Limestone are in 

 most cases still articulated by the ligament, and separated merely 

 to the extent naturally resulting from the decay of the animal. 



The great accumulation of marine life indicated by the Shell- 

 Limestone does not seem to have been adversely affected by the 

 presence of magnesian salts. Every sample of the typical, un- 

 segregated, often profusely fossiliferous rock, taken from all 

 horizons, that I have analysed, shows a uniformly high percentage 

 of magnesium carbonate, ranging generally from 40 to 45 per cent. 



By whatever means the magnesia gained access to a rock such 

 as the Shell- Limestone, or its equivalent beds on the eastern or 

 western side, which, when unsegregated, show a similar compo- 

 sition, its presence is undoubtedly an original and contemporaneous 

 feature of the formation. 



The phases of the Upper Shell-Limestone exposed in the 

 Hartlepool area may be tabulated as follows, in descending 

 order, with their visible thickness in each section and the most 

 obvious faunal features. The sequence must not be taken as a 

 constant one, as the Upper Shell-Limestones probably thicken 

 southwards : — 



Shell-Limestones exposed in the Hartlepool Area. 



Thickness in feet. Faunal Features. 



f Bracbiopod species, 2. 



Uppermost Bedded Shell- "^ | Stroplialosia very rare. 



Limestones. L CO J Intense dwarfing of fauna. Gastero- 



(Upper end of Hesleden [ j poda, Bakevellia antiqua, and 



Dene.) J Fpithyris very common but dwarfed 



1^ and variable, 

 f Bracbiopod species, 3. 



Shell-Limestones with inter-"} j Stroplialosia rare. 



bedded Conglomerates. L 40 J Typical middle forms very dwarfed. 



(Blackball Rocks and lower [ 1 Fpithyris common, but localized, 



end of Hesleden Dene.) J Bakevellia ceratophaga very 



(_ common. 



o id. iit- > C Bracbiopod species, 4. 



Bryozoonal Shell-Limestones.^ j Dwarfm g less apparent. Stroplialosia 



(Castle- Eden Dene; Ivy- ^ 80 ^ Common. Bryozoa as reef- 



Rock section.) J L bulkier*. 



Bryozoonal Shell-Limestones."} f Bracbiopod species, 6. 



(Castle-Eden Dene: j> 20 -^ Camarophoria schlotheimi and 



Deneholme section.) J (^ gonotreta multiplicata appear 



In Blackhall-Colliery Sinking the full thickness of Shell-Lime- 

 stone (about 335 feet) was encountered, and proved to exist 

 beneath the surface in the district. 



The upper part of the Shell- Limestone, which alone occurs in 

 the outcrops in the area under consideration, with its impoverished 

 and dwarfed fauna, was clearly indicated as overlying the typical 

 highly-fossiliferous rock, with profusion of brachiopods and other 



