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



The Lower Limestones reach a thickness of over 250 feet in the 

 south of the county, and usually appear as a series of bedded 

 dolomites or dolomitic limestones, with which are interbedded 

 lenticular masses of highly calcareous grey limestone. They thin 

 westwards towards the shore-line of the Permian sea, which lay on 

 the flanks of the Pennine ridge, then partly developed. The 

 dolomitic limestones reach a thickness of over 100 feet, but some- 

 times the two types are interbedded in irregular bands. The 

 calcareous rocks are locally, as in Thickley Quarry, composed of 

 fossils in a well-preserved state, but inorganic limestones with an 

 occasional specimen of Productus horridus occur in Paisby Hill 

 Quarry, over 100 feet in thickness. The dolomitic beds of the 

 Lower Limestone are chemical precipitates, and the calcareous beds 

 represent conditions when the dolomitic precipitation was arrested. 

 When followed northwards the Lower Limestones maintain their 

 thickness as far as Sunderland, but they thin rapidly north and west 

 of this area; in some sections it is, however, difficult to estimate 

 their original thickness in these directions, as their upper beds have 

 been disturbed and displaced along the major thrust-plane of this 

 deformed area. They are, however, only about 30-40 feet thick in 

 Frenchman's Bay, South Shields. Many of the geodes in this lime- 

 stone now lined with calcite originally contained gypsum or 

 anhydrite. Pockets containing these minerals have been proved in 

 the borings in South Durham. 



-WLlt 



FlG. 1. — Diagrammatic section across the Bryozoa Keef to show the relations 

 of the different divisions of the Magnesian Limestone to it. L, Lower 

 Limestones. Mi, the western equivalents of the Eeef, fossiliferous and 

 unfossiliferous dolomites, granular dolomites, dolomitic oolite, and 

 calcareous segregated rocks. E, the Bryozoa Beef. M2, the eastern 

 equivalents of the Beef, unfossiliferous dolomitic limestones, dolomitic 

 oolite, calcareous segregated rocks, breccias, and pseudo-breccias. V, 

 interbedded fossiliferous breccia — the Vor-reef. FL, the Flexible Limestone. 

 C, the Concretionary Limestone. F, slickensided slip-planes. The length 

 of the section is about two miles. The Bryozoa Beef is over 300 feet thick. 



The Middle Limestone is of special interest as it exists under 

 three distinct facies, and it was undoubtedly this fact that was one 

 of the main difficulties in understanding the stratigraphy of the 

 Magnesian Limestone. Punning right through the division is the 

 shell-bank of the Kryozoa P<ef, usually called the Shell or 

 Fossiliferous Limestone. It is generally about a mile across, and its 

 total original thickness was over 300 feet. In the north it has been 

 highly denuded, appearing as a series of rounded knolls, and is 



