Vol. 70.] OF DUEHAM MAOXESIAX LIMESTONES. 259 



VII. Summary of the General Conditions of Deposition. 



The Marl Slate and Yellow Sands have been so thoroughly- 

 described by Prof. G-. A. Lebour 1 and others, that there is little 

 for me to add, except a few words about the Marl Slate : it under- 

 lies the whole Magnesian Limestone area of Durham. It is from 

 3 to 8 feet thick, and is variable in composition. It represents a 

 widespread area of tranquil deposition of very thinly-laminated 

 dolomitic and calcareous marl, into which dried, distorted, and partly- 

 decomposed fishes with occasional amphibians and reptiles, plant- 

 remains, coprolitic matter, and shells were periodically drifted,, 

 probably from estuarine areas in process of desiccation near land. 

 I do not think that it is itself a lagoon -deposit, since neither 

 sun-cracks, ripple-marks, nor false-bedding have been detected in 

 it. Its equivalent, the G-erman Kupferschiefer, is regarded as a 

 deep-water deposit in the relative sense of the term. 



The calcareous beds with a brachiopod fauna, which occur near 

 the base of the Lower Limestones in the south-western Durham 

 area, are due to conditions of deposition during which the process- 

 of dolomitic precipitation or of dolomitization was temporarily 

 arrested. The calcareous bed at Thickley Quarry is a lenticular mass 

 interbedded with the dolomite. Examination of partly-decalcified 

 surfaces shows that the rock is very largely composed of small 

 shell-fragments, although thin sections reveal very little structure. 



The 60 feet of calcareous beds near the base of the Lower Lime- 

 stone at Raisby Hill are probably of similar origin, but traces of 

 organic structure are less apparent. Similar beds occur near 

 Masham in North Yorkshire. Otherwise, the Lower Limestone in 

 all its divisions shows a highly dolomitic composition. In Blackhall 

 Sinking, all the beds were dolomitic throughout the 250 feet of 

 the Lower Limestone. 



Owing to the comparative absence of soluble sulphates, the 

 segregational changes have been slight, compared with those in the 

 overlying beds ; and the Lower Limestone retains to a greater degree 

 than any other division of the Magnesian Limestone its original 

 condition of deposition. Oolites only occur in this division near 

 the Tees, where the Durham facies merges into the more littoral 

 facies of Yorkshire, in which area oolites are conspicuously developed 

 in the Lower Limestone. 



Increasingly abnormal conditions are at once apparent in the 

 Middle division, with its long meandering line of reef assuming a 

 true knoll-like aspect on its eastern flank, made up of an enormous 

 accumulation of now-dolomitized organisms, and in their progres- 

 sive extinction, species after species, on ascending in the reef. The 

 reef was probably deposited about 10 or 12 miles away from the 

 land. 



The Middle Beds flanking the eastern and western sides of the 

 reef, in which most of the cellular structures occur, have already 

 been described. 



1 ' The Marl-Slate & Yellow Sands of Northumberland & Durham ' Trans. 

 N. Eng. Inst. Min. Eng\ vol. liii (1902-1905) p. 18. 



