260 MR. C. T. TBECHMANN OX THE LITHOLOGY [June 1914, 



The Flexible Limestone is confined to the northern area, east of 

 the reef. It represents a largely detrital deposit, recalling in some 

 ways the conditions of the Marl Slate. 



The beds in which concretionary botryoidal structures are de- 

 veloped, were probably of a rather more calcareous nature than the 

 higher beds of the Upper Limestones. 



The eastward retrogression of the Upper Limestones with respect, 

 probably to the Middle, and certainly to the Lower Beds, deserves 

 notice, because it has some bearing upon the composition and 

 lithology of the beds themselves. In South Durham the western- 

 most outlier of the Lower Limestone occurs at least 17 miles to 

 the west of the Upper Limestones, which are here thinning rapidly 

 and changing in character. An examination of the Geological 

 Survey maps of Yorkshire and Nottinghamshire, where the Upper 

 Limestones are mapped, shows a similar retrogression. In Durham 

 the upper beds, esjDecially the concretionary division, thicken on 

 passing eastwards from the reef, while the thickening of the 

 Magnesian Limestone as a whole in Nottinghamshire, on retreating 

 both eastwards and northwards from the shore-line, is noticed by 

 several authors. 



This retreat of the Upper Limestones is explicable, not by any 

 theory of uplift, but by that of the shrinkage of the limits of the 

 Permian sea, following on increasingly arid conditions with con- 

 sequent evaporation and failure of the water-supply. The highly - 

 dolomitic nature of the Highest Limestones in Durham points to 

 the latter effect, while the great development of oolites in these 

 beds may indicate the proximity of tidal or surface-movement of 

 the water. 



As regards the eastward thickening of the Upper Limestones on 

 retreating from the shore-line, if the Magnesian Limestone be in 

 reality for the greater part a deposit from the waters of the sea, 

 then the greater the volume of overlying water, the greater will 

 be the quantity of material deposited, especially from the very 

 concentrated waters of the Upper Limestones. The consequence 

 will be a thickening of the deposits on reaching deeper water. 



Regarding the hidden salt-deposits of South Durham little can 

 be said, except that their Permian age was established by R. Howse, 1 

 on the discovery of the three characteristic Upper Zechstein fossils, 

 in an intensely dwarfed condition, in a dolomite-band closely asso- 

 ciated with the Salt Measures. 



Prof. Lebour 2 remarks upon the limited westward extension of 

 the salt-deposits. They represent, in fact, the final phase in the 

 sequence of events traceable throughout the Magnesian Limestone, 

 and are probably a fragment of the western margin of the great 

 German Upper Zechstein salt-field. In the records of borings 

 there is every indication of a perfect upward passage into the Bunter 

 rocks. 



1 Nat. Hist. Trans. North. & Durham, vol. x, pt. 2 (1890). 

 - ' Handbook to the Geology & Natural History of Northumberland & 

 Durham ' Brit. Assoc. (Newcastle) 1889, p. 31. 



