IKK I'KRMIAN FORMATION'. ^07 



fresh water and less sediment into the Permian lagoons than previously, 

 and, ceasing to overflow their hanks, would no longer carry away the 

 quantities of laud plants, brought down during the Marl Slate stage, (a) 

 Subsidence continued during this stage, accelerating in amount north- 

 wards, through Yorkshire to Durham. This movement, by diminishing 

 the altitude of the catchment area, while increasing the area of 

 evaporation would, to a certain extent, tend to promote the conden- 

 sation and precipitation of mineral matters in the land-locked waters 

 of the period. The southerly intercalation of sedimentary beds 

 between calcareous deposits, viz., the Middle Marls and Sandstones, 

 may have been due to the influx of rivers from the land lying on the 

 south, which, like that on the west, appears to have remained un- 

 submerged during the whole of the Permian period. The presence of 

 gypsum, however, in these beds shows that chemical action still 

 continued actively at work. This chemical deposit, as well as the 

 ripple marks and suncracks met with in these beds, demonstrate the 

 extreme shallowness of the waters in which they were formed. The 

 Upper Permian Marls may perhaps be similarly accounted for, and 

 possibly indicate the approaching termination of salt lake conditions. 



Here, then, the record of the physical succession of events in the 

 North-east of England during Permian times abruptly terminates. 



While the Magnesian Limestone was forming, the fauna appears to 

 have been limited in variety, and, as a rule, in number of individuals, 

 except for a time in the Durham district, where the waters appear to 

 have been, on the whole, somewhat deeper, and therefore sweeter 

 than further south in South Yorkshire and Notts. This fauna is, 

 indeed, distinctly marine, but the species are comparatively few in 

 numbers, and, as a rule, dwarfed in size, such as we might expect to 

 find in an inland lake or lagoon, once for a time connected with, but 

 that had since become cut off from communication with the open sea. 

 On this hypothesis, however, the distribution of this dwarfed marine 

 fauna — almost wanting in the lowest beds, (Marl Slates,) more 

 numerous in the Lower Limestone, (especially of Yorkshire and Durh am,) 

 and in Durham still more numerous in the middle portion of the 

 Magnesian Limestone — compel us to suppose that between the open 

 sea and the westerly limits of the Permian basin of the North-east of 

 England, there must have been one or more intervening lagoons, across 

 which very few forms of animal life succeeded in making their way in 

 Marl Slate times. Into those regions, however, in Lower Limestone 

 times a certain number of already dwarfed animals had succeeded in 

 penetrating. In Middle Limestone (= upper part of Lower Lime- 

 stone ?) times, a larger migration took place into the Durham area. 

 Nearly half of these had lived in Yorkshire during Lower Limestone 

 times, but very few succeeded in making their way into Notts, where, 



(a) Hence also the change in the colouration of the rocks ; the iron, in the 

 absence of these decaying organic matters, being now thrown down as the 

 yellow hydrated ferric oxide, or a red coloured compound of iron, instead of as 

 the colourless or grey ferrous carbonate, 



