Vol. 69.] IN THE MAGNESIAN LIMESTONE AT HARTLEPOOL. 20'S 



forms, which is so characteristic of the northern area of the county. 

 This upper or Productus-iiee phase, so far as I could ascertain, had 

 a thickness of about 150 feet in Elackhall Sinking. 



The fauna of the uppermost Shell-Limestones approaches in 

 character that of the brachiopod-free Upper Limestones. It is 

 characterized by (a) the comparative absence of brachiopoda, 

 (6) the abnormal dwarfing of many typical Shell-Limestone forms, 

 (c) the numerical profusion of certain genera Avhich are charac- 

 teristic of the Upper Limestones {Turbo and other small gastero- 

 poda, Myalina, Schizoclus, etc.). 



The increasingly unfavourable conditions that prevailed towards 

 the top of the Shell-Limestone bring about the gradual extinction of 

 all the typical brachiopod genera, except Epithyris and Strophalosia 

 (many of the largest and most conspicuous forms, such as Spirifer 

 alatus and Athyris pectinifera, being the first to disappear, followed 

 shortly by Productus and the large CamaropJiorice). The more 

 adaptable lamellibranchs persist, though many in a dwarfed con- 

 dition. Pleurophorus survives the ordeal, although dwarfed to 

 about a third of its normal size, and persists throughout the Upper 

 Limestones. The gasteropod genus Turbo is comparatively un- 

 affected, as its great numerical abundance in some beds in Hesleden 

 Dene and at Blackball Rocks testifies. 



The profusion of small littorinoid gasteropocls and mytiloid 

 lamellibranchs might be considered as evidence of influence of 

 littoral conditions in the upper beds of the Shell-Limestone. 

 The beds of small gasteropoda may be taken as evidence of 

 shallowing water over the reef, 1 but neither the composition of the 

 rock which maintains its uniform, highly magnesian character, nor 

 any increase in the insoluble residue, indicate any change from the 

 conditions prevailing in the typical lower beds of the Shell- 

 Limestone. 



The reason for the decadence of the fauna is more probably to 

 be sought in the increasing amount of sulphates in the sedimenta- 

 tion, following upon the gradual failure of the conditions which 

 gave rise to the formation of the Shell-Limestone. This view 

 seems to be further favoured by the great amount of bedding 

 (regular and irregular) and internal deformation, at present 

 observable in all sections in the Upper Shell-Limestones in the 

 district, as also the association with them of unfossiliferous beds 

 containing the cavities, presumably left by sulphates, already 

 described. 



VI. Typical Sections and Exposures in the Hartlepool Area. 

 (1) Hartlepool Foreshore. 



The cliff-section is now greatly obscured by the sea-walls, but 

 the main succession may be studied and Upper-Limestone fossils 

 collected at various points at low tide. 



J. W. Kirkby, Q. J. G. S. vol. xiii (1857) p. 218. 



