Vol. 6g.^\ ANHYDRITE-MASS IN THE MAGNESIAN LIMESTONE. 193- 



III. Boeings in the Magnesian Limestone where it is protected- 

 bi- Hed Beds. 



The great boring at Seaton Carew was described in detail by 

 E. Wilson, 1 who stated that no fossils were detected in the cores, 

 so that no opinion can be formed as to the horizon of the lime- 

 stones met with. The limestone here occurs nearly 200 feet 

 higher up than at West Hartlepool Cement- Works, and probably 

 has a northward dip. The section is important in connexion 

 with the present paper, as revealing a mass of ' anhydrite with 

 beds of dark-blue shale and gypsum,' 35 feet thick, at a depth 

 of 84 feet from the top of the Magnesian Limestone, which here 

 attains the greatest recorded thickness of 878 feet. It is note- 

 worthy that the limestone is gypsiferous throughout this great 

 thickness. 



A critical examination of this boring, so far as the details 

 recorded may be reliable, shows the aggregate proportions of the 

 formation to be as follows : — 



Feet. 



Sulphate-free limestone 480 



Limestone impregnated with gypsum and anhydrite 358 



Pure anhydride and gypsum 35 



At 140 feet from the bottom of the limestone is a thickness of 

 260 feet of limestone free from gypsum, -which might correspond 

 to part of the Shell-Limestone, but no fossils are recorded. I 

 have very little doubt that they would have been found, if a search 

 had been made. 



With this boring another may be contrasted : namely, that sunk 

 at Whitehouse Farm near Norton in 1892, 2 about 7 miles south- 

 west of the Seaton-Carew boring, where the thinnest recorded 

 complete section of the Magnesian Limestone occurred, measuring 

 only 299 feet in thickness. No fossils are recorded. Whichever 

 division of the formation may be represented by this rock, the fact 

 of importance in connexion with this discussion is the presence of 

 large interstratified beds of anhydrite and gypsum, having an 

 aggregate thickness of 84 feet. They commence about 85 feet 

 from the top of the Magnesian Limestone, the most considerable- 

 individual bed being 37 feet thick. The sulphate-free rock in this 

 boring has an aggregate thickness of about 160 feet, while the beds 

 of limestone impregnated with anhydrite and gypsum aggregate 

 about 56 feet. 



The presence of large quantities of anhydrite and gypsum in the 

 Magnesian Limestone is a usual feature wherever that formation 

 is covered by a protective layer of comparatively impervious 

 material as in South Durham, and undoubtedly represents a more 

 original and unaltered condition of the formation. 



This fact encourages the idea that the mass of anhydrite beneath 



] 



1 Q. J. G. S. vol. xliv Q88S) p. 781. 



2 T. Tate, ibid. vol. xlviii (1892) p. 488. 



