A HISTORY OF DURHAM 



with which this paper concludes [a collection of detailed sections], seems fairly ob- 

 vious. 



(i) A mass of sand, probably chiefly derived from the waste of the Carboniferous 

 Sandstones which formed so large an area of the then land-surface to the west, occu- 

 pied a broad tract of coast from somewhere to the north of Hartley, in Northumber- 

 land, to Yorkshire and still farther south, narrower in the north than in the south. 

 This sand was a beach at the coast line and a desert of blowing dunes elsewhere. 

 Rivers, sluggish, and probably inconstant (changing their course as do the channels in 

 a delta), wound their way to the sea across this sandy tract, and added to the irregu- 

 larity of its surface.' The deposition of calcareous and magnesian mud in the thinly 

 bedded layers which betoken tranquil deposition followed, due partly to silting from 

 landwards and from tidal irruptions from seawards most probably in a chain of coastal 

 lagoons. This was accompanied by a downward movement of the coast line and the 

 gradual merging of the lagoons into the sea proper when the Magnesian Limestone, 

 with its curious fauna — a marine fauna checked in its existence by the unfavourable 

 chemical composition of the Permian sea water to which the rock owes its dolomitic 

 char.-icter — was deposited. This view is strongly confirmed by the occasional excep- 

 tions to the rule that the Marl-slate precedes the Magnesian Limestone proper which 

 already have been referred to, such exceptions (where limestone occurs beneath the 

 so-called Slate) being obviously the result of local accidental breaches of the bars sepa- 

 rating the lagoons from the sea.^ 



The Marl-slate referred to in this extract is the next Permian 

 division above the Yellow Sands. Whether the latter can in any real 

 sense be said to represent the much more largely developed Rothliegendes 

 of the German Dyas may be regarded as doubtful in the absence of 

 palsontological evidence. That the thin Marl-slate is the equivalent of 

 the Kupfcrschiefer is however open to no doubt, although in this 

 country seldom more than a yard in thickness this formation of impure 

 calcareous slabby beds of grey or brownish colour contains a storehouse 

 of fossils which sufficiently attest its exact stratigraphical horizon. 

 Besides shells such as Nautilus frcieslehem, Lingula credmri^ Discina konincki 

 and Myalitm hausmanni, and plants (imperfectly preserved but capable of 

 identification) such as Neuroptcris huttotiiana, Cauloptcris (?) sehiginoides 

 and Polysphonia (?) stertihergiana, this deposit is a true fish bed and yields 

 extraordinarily perfect specimens, usually as entire individuals, of such 

 vertebrates as PaUeoniscus, Dorypterus, Acentropus, Pygopterus, Acrolcpis, 

 Ccelacanthus, Platysomus — represented by many species, as well as 

 amphibians and some true reptiles such as Protcrosaurus. In the 

 county it is at Claxheugh, Deaf Hill, Middridge near Sliildon, Thickley, 

 and Ferryhill that some of the most remarkable specimens have been 

 found. 



The next, and much the most fully developed division of the 

 Permian, following, with perfect conformity over the Marl Slate, is 

 the Magnesian Limestone, which in Britain is nowhere so thick or so 

 splendidly exposed for study as in the cliff sections of Durham and 



I 



' The 1.11c Prof. A. H. Green w:<3 of opinion that the quicksands (th.it is, our Yellow Sands) .ira 

 the dcltaj of tlic strc.ims which emptied themselves into the I'crmi.Tn inl.ind se.i [Geol. M<]g. [1S72J, 

 IX. 101). The entire alwcnce of fossil rcm.iins, the form of the grains, and the nature of the cross 

 l>cdJinK, »ccm to point rather to wind as the final distributor of tlic sand, though Prof. Green's view 

 may quite well be accepted for their first accumulation. 



* Tram. Inii. Min. Engineers, 1903. 



18 



