Vol. 69.] IN THE MAGNESIAN LIMESTONE AT HARTLEPOOL. 195 



Blackhall Sinking the thin bed of Marl-Slate together with the 

 Yellow Sands were to all appearances undisturbed and in normal 

 condition, in striking contrast with the great mass of deformed 

 carbonates lying above. 



Slickensided surfaces have been noticed in several instances. 

 They occur between the masses of Shell-Limestone conglomerate 

 at Blackhall Hocks, and can be seen on a small scale in the 

 brecciated gypsiferous limestone beneath the anhydrite-mass al 

 Hartlepool, where the gypsum seems to have acted as a lubricant ; 

 but these instances are purely local, and are such as might have 

 been produced by movements set up in the mass of the rock through 

 reduction in volume by solution. 



"Whether the more extended movements which have recently 

 been described x from the Sunderland area are due to the same or 

 to independent causes must be left as an open question for the 

 present. 



The discrepancy between the thickness of the protected limestone 

 at Seaton Carew (878 feet) and that of typical sections of the 

 unprotected formation north of the .fault which include all the 

 divisions (Blackhall Sinking— 688 feet, Hart Boring— 708 feet), 

 would seem to point to a very considerable reduction in volume 

 on removal of the sulphates ; though it would appear to be necessary 

 to hold this view with reserve, owing to the possibility of the fault 

 having been active in Permian times. 



IV. The Former Presence of Soluble Constituents 



IN THE MAGNESIAN LlMESTONE. 



The former presence of soluble constituents, presumably sulphates,, 

 can be inferred from several features observable in the formation 

 in this and other areas. 



(1) The intense porosity of many beds, both in the Upper and in 

 the Middle Limestones, which, from the existence of plentiful casts 

 of fossils, are shown to be practically unaltered in volume. The 

 highly magnesian character of these beds further indicates their 

 original condition. 



(2) The presence of hollow or negative cavities or spaces left by 

 crystals or aggregates of crystalline material in many beds. They 

 are most apparent in the well-stratified, unfossiliferous beds of the 

 middle division, overlying, or otherwise associated with, the Upper 

 Shell-Limestones ; but I have also found them sparsely distributed 

 in some of the fossiliferous beds. When very numerous, causing 

 the rock to become extremely porous, they lead in some cases to a 

 structural collapse with development of the usual hard, calcareous, 

 segregated breccias. When replaced by calcite, the crystalline 

 aggregates appear as stellate structures on broken surfaces. 



1 D. Woolacott, ' Stratigraphy & Tectonics of the Permian of Durham 

 (Northern Area) ' Q. J. G. 8. v 3 l_. lxvii (1911) pp. 312-13. 



