Vol. 69.] IN THE MAGNESIAS LIMESTONE AT HARTLEPOOL. 217 



Till. Summary op Conclusions. 



(1) The Magnesian Limestone, in the district here described, in 

 its original condition of deposition contained very large quantities 

 of calcium sulphate, as shown by several borings entering or 

 piercing the Magnesian Limestone where it underlies the Red Beds 

 or other protective covering. 



(2) This calcium sulphate has been completely removed by the 

 action of percolating waters, wherever the Magnesian Limestone has 

 been divested of its protective covering of comparatively impervious 

 Permian and Triassic red marls and the associated beds. 



(3) A mass of anhydrite, apparently by reason of its exceptionally 

 large size, has escaped complete removal ; and, because its denuda- 

 tion and erosion were more rapid than was the case with the 

 surrounding Magnesian Limestone, it has given rise to the ' slack ' 

 or valley which now accommodates the harbours of Hartlepool and 

 "West Hartlepool. 



(4) This mass of anhydrite overlies a gypsiferous (protected) 

 Magnesian Limestone containing fossils of the Middle Division 

 (Shell-Limestone), and so must represent the time-equivalent of 

 apparently part of the Middle, and of a great part of the Upper 

 Limestones. 



(5) Evidence goes to show that the sulphates in the Magnesian 

 Limestone were originally deposited as anhydrite, and that this an- 

 hydrite was converted into gypsum before removal. The bands of 

 limestone which are interbedded with anhydrite towards the base 

 of the anhydrite-mass represent Magnesian Limestone in its original 

 condition. The gypsiferous limestones which occur beneath the 

 anhydrite-mass at Hartlepool or other areas represent an inter- 

 mediate stage between the unaltered rock and the completely 

 gypsum-free limestones at the surface. 



(6) The removal of soluble sulphates from the formation has 

 brought about a degradation and settling-down, with a consequent 

 decrease in volume of the Magnesian Limestone, resulting in 



(a) A mechanical collapse of parts of the strata with consequent brecciation ; 

 and (b) an intense porosity in those beds where the sulphates impregnated, or 

 existed in close association with, the carbonates. 



(7) The porous condition in which parts of the formation are left 

 after removal of the sulphates lays it more open to processes of 

 segregation and differentiation of . the two carbonates, with pro- 

 duction of hard calcareous breccias associated with powdery material 

 rich in magnesium carbonate, which are an obvious feature of the 

 formation in its present condition. The subsequent removal by 

 water (by solution or by mechanical means) of the powdery material 

 brings about the dedolomitization of parts of the formation on a 

 more or less extensive scale. 



(8) Analysis of obviously less-altered portions of the formation 

 goes to show that the Middle and Upper Magnesian Limestones 

 were originally deposited in a highly dolomitic condition. "While 

 the variability in composition of the Lower Limestones, where 



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