488 Dr. D. Woolacott — Magnesian Limestone of Durham. 



are so well exposed along the middle part of the Durham coast, and 

 which often appear as breccias and pseudo-breccias, so that they 

 have been called the Brecciated and Pseudo-Brecciated Limestones. 

 On the west it is represented by a stratified series of scantily 

 fossiliferous dolomites, granular dolomites, dolomitic oolites, 

 and segregated calcareous rocks. This series only occurs fully 

 developed under the drift in the south of the county ; in the north 

 it has been highly denuded. It can be studied in quarries near 

 Silksworth and in Haswell Quarry. Occasional inroads of the fauna 

 of the reef must have taken place into the lower beds of this facies 

 when they were laid down, but some of the fossils in them were 

 probably drifted in from the reef. 



XZo 



FlG. 2. — Curve showing the number of invertebrate species recorded in the 

 different divisions of the Durham Permian. A curve showing the number 

 of individuals would be of similar form with the height of the apex in the 

 Bryozoa Beef enormously increased. A-B, the Yellow Sands. B, the 

 Marl Slate. B-C, the Lower Limestones. C-D, the Bryozoa Beef. 

 D-E, the Concretionary Limestone. E-F, the Boker and Hartlepool 

 Oolite. F-G, the Upper Bed Beds with salt. 



The base of the Upper Limestone is marked by the Flexible Lime- 

 stone, which I have traced all over the northern area, and the 

 equivalent of which Trechmann has proved to occur in the south of 

 the county. It marks a decrease in the depth of the Permian sea, 

 recalling the Marl Slate in its laminations, occasional fish-remains, 

 and high percentage of impurities, and seems to have been deposited 

 under similar tranquil conditions in fairly deep off-shore waters. 



