Vol. 67.] PERMIAN TO THE TRIAS IN NOTTINGHAMSHIRE. Ill 



Permian is thinning southwards : the upper member disappearing 

 first and the lowest last. There is no doubt that the Permian 

 deposits do thin southwards, and also (as is shown by borings) 

 westwards, as does the Trias above. The explanation is, however^ 

 insufficient to account for the facts. While the Trias as a whole thins 

 southwards, its lower members thin northwards : it is suggestive,, 

 at the least, that as the Permian Marl and Upper Magnesian 

 Limestone develop northwards, so does the Lower Mottled Sand- 

 stone diminish, and, after this has entirely disappeared, the Pebble 

 Beds above lose in importance and finally disappear also, leaving 

 the Keuper resting directly upon the fully developed ' Permian.' 



The conditions of deposition of the Permian and Triassic rocks is 

 still largely a mystery. It is, however, clear that the magnesian 

 limestones and the Keuper were formed in a Caspian Sea, and that 

 the Bunter is probably non-marine. 1 



The thin Permian of Nottingham, which is distinctly a littoral 

 formation, can scarcely be the time-equivalent of the thick deposits 

 of Durham, which consist of 800 feet of limestone. But the Marl 

 Slate with its fossils occurs at the base of the series in both places,, 

 and therefore the upper part of the Durham Limestone is newer than 

 any part of the Nottingham Permian, a fact corroborated by such fossil 

 evidence as exists. Now, the Nottingham Permian was immediately 

 followed by Bunter, so that the upper part of the Durham Mag- 

 nesian Limestone is the chronological equivalent of some part of the 

 Nottingham Bunter : that is, both are Permian or both are Trias. 



We know that the early Zechstein sea had its southern limit 

 at Nottingham, and its western limit not far to the west of the 

 present outcrop : hence the assumption of a gentle uplift acting 

 during Zechstein times, and causing the coast-line to migrate in a 

 north-easterly direction, would account for Bunter being formed 

 towards the south, perhaps by torrential floods on the flat coast- 

 lands, contemporaneously with the continued formation of limestone- 

 off the coast. Meantime the sea-bottom was slowly subsiding in 

 the Durham area, allowing of the formation of a thick mass of 

 shallow- water limestone. There would, therefore, be a slowly 

 rising area in the south and a slowly sinking area in the north ;. 

 while the line about which the movement turned migrated in a 

 north-easterly direction. It appears that, at the end of the period,, 

 the sea, always of a Caspian character, became increasingly salty. 

 This is indicated by the poverty of the fauna and dwarfed nature of 

 the fossils in the Upper Limestone, 2 and by the occurrence of rock- 

 salt and gypsum in Durham, towards the end of the period. The 

 Keuper, which followed, indicates a slight inroad of the sea, which 

 was still, however, very salt, for dolomite, gypsum, and rock-salt 

 occur in this series also. 



It is probable that, in at least the southern half of the district,, 

 there occurred a slight check to the slow steady rise of the area, 

 during Permian times. This is indicated by the slight unconformity 

 between the Lower Limestone and the Middle Marl, which has 



1 T. G. Bonney, 1908, p. 337; and J. Lomas, 1907, p. 511. 



2 J. W. Kirkby, 1861, p. 316. 



