112 ME. R. L. SHERLOCK ON THE RELATIONSHIP OP THE [JFeb. I9II, 



been seen at intervals between Nottingham and Tadcaster. The 

 Marl indicates somewhat deeper water than the limestone (which 

 at Mansfield shows reptilian footprints and ripple -marks), and the 

 passage-bed above it is very like the Keuper Waterstones in 

 appearance, and therefore probably had a similar origin. Now, the 

 Waterstones seem to indicate shallower water than the Keuper 

 Marl above: for they are composed of coarser sediment, and the beds 

 are not only ripple-marked but even sun-cracked in places; and 

 the Permian Marl is lithologically very like the Keuper Marl. It 

 appears, therefore, that, after the slight depression which ended the 

 formation of the Lower Limestone and caused the Marl to be 

 deposited, the steady rise set in again immediately. 



The close of the Permo-Bunter period was brought about by a 

 change in these movements. In the south, the Bunter was covered 

 by the sea, for the Keuper Waterstones were deposited in water ; 

 and there appears to be a slight unconformity between them and 

 the Bunter. In the north, there may or may not have been a 

 change of movement, since both Waterstones and Magnesian 

 Limestone are shallow-water deposits. The difference between 

 them may be merely a question of the supply of sediment, for, as 

 was remarked above, the Waterstones contain much dolomite. 

 All that seems necessary, therefore, to bring about the change from 

 Permo-Bunter to Keuper conditions was the sinking of the southern 

 area below a very shallow sea, with the consequent alteration of 

 currents in the north. The Waterstones extend over the whole 

 of the north-eastern basin ; and slow subsidence, causing slight 

 deepening of the water, would bring about the gradual change 

 which caused Keuper Marl to follow the Waterstones imperceptibly. 

 All this time, however, the sea remained of Caspian character, 

 until, at last, the open sea found an entrance and the Rhaetic beds 

 were formed. 



These considerations lead to the following correlation-table : — 



South Nottinghamshire. South Yorkshire. Durham. 



Keuper Marl. Keuper Marl. Salt 7 _ 



r Jveuper. 

 Keuper Waterstones. Keuper Waterstones. Measures. ) 



Probable slight unconformity. 

 Pebble Beds. "1 f Pebble Beds, "j 



Lower Mottled Sandstone ! J Upper Marl (local). 



( and Passage Beds.) J j Upper Magnesian Lime- ^ = ^ MiddleMagnesian Lime- 

 Middle Marl. Middle Marl. 



Lower Magnesian Lime- Lower Magnesian Lime- 



stone, stone. 



Upper Magnesian Lime- 

 stone. 



I 



Lower Magnesian Lime- 

 stone. 



Basement Beds (local). Basement Beds (local). Basement Beds (local). 



It is to be noted that the Basement Beds are local and may not be strictly 

 ■contemporaneous everywhere. Kirkby (1861, p. 317), in fact, regarded the 

 Marl-Slate of Durham as contemporaneous with part of the Lower Magnesian 

 Limestone of South Yorkshire. 



