Br. D. Woolacott — Magnesian Limestone of Durham. 453 



and a statement of our present knowledge of these rocks. I am able 

 to do this more readily as part of the southern Permian has 

 been described and much careful work done on the lithology and 

 composition of these rocks by Dr. Trechmann l (to whose work 

 I must record my indebtedness in the preparation of parts of this 

 paper), and two borings that have lately been put down have given an 

 opportunity for the examination of one of the least known facies of 

 the Middle Limestones. 2 



The uneven floor of the broad syncline of the Northumberland and 

 Durham Coalfield was covered by a series of dunes on the edge of the 

 Permian sea, which was advancing from the east. These were 

 gradually planed down by the incoming waters, the Yellow Sands 

 being formed. This deposit usually consists of large rounded grains 

 set in a finer angular-grained matrix. The basin in which they were 

 laid down extended far to the east, but on the west it was bounded 

 by the Pennine area, and on the south by an anticline of Lower 

 Carboniferous rocks which then lay across the south of Durham. 

 Fragments of the latter rocks with derived encrinite stems occur in 

 the Yellow Sands in the south of the county, proving that this ridge 

 was being denuded when these sands were being deposited. On 

 these reassorted sands a finely laminated calcareous and dolomitic 

 mud — the Marl Slate — was laid down under tranquil conditions in 

 fairly deep off-shore waters. Eventually the southern ridge was 

 covered by deposits of the limestone facies and the sea thus 

 overspread the country to the south, and deposition went on 

 continuously from Durham to Central England. 



The Magnesian Limestone, which reached a thickness of about 

 1,000 feet, was deposited in an inland sea which was gradually 

 drying up, so that conditions were brought about which led to the 

 gradual precipitation of the dissolved salts. The strata originally 

 varied from nearly pure calcareous rocks to pure dolomites, along 

 with which were beds, veins, and intercalations of gypsum and 

 anhydrite, and in the later formed rocks of marls, marly sandstones, 

 and rock salt. The main mass of the Magnesian Limestone thus 

 consists of (a) thick and thin beds — often lenticular — of nearly pure 

 calcium carbonate, (b) beds of nearly pure dolomite, (c) beds composed 

 of mixtures in varying proportions of dolomite and calcium carbonate 

 — these are the dolomitic limestones consisting of dolomite and 

 interstitial calcite, and (d) (where they have not been removed) beds, 

 veins, etc., of calcium sulphate. 



Irregularly distributed throughout the limestone are various 

 substances, some of which are due to original deposition, while others 

 are of secondary origin. Among such substances are alumina, chert, 

 iron oxides, manganese dioxide, grains of quartz (both of detrital and 

 non-detrital origin), and, in a lesser degree, of mica, tourmaline, 

 zircon, etc. In geodes and cavities crystals of calcite are common, 

 while dolomite is only occasionally found. Ankerite, fluorite, galena, 

 iron pyrites, malachite, heavy spar, and limonite (pseudomorphs 



1 Q.J.G.S., vol. lxix, pp. 184-218, 1913, and vol. Ixx, pp. 232-65, 1914. 



2 " Borings at Cotefield Close and Sheraton" : Geol. Mag., March, 1919, 

 pp. 163-70. 



