Dr. D. Woolacott — Magnesian Limestone of Durham. 457 



Similar conditions of deposition held good through a protracted 

 period, during which some 600 to 700 feet of rock were formed. 

 The Permian Sea became shallower, marls, marly false-bedded sand- 

 stones and sodium chloride were deposited, and finally all the calcium, 

 and magnesium carbonates having been laid down, the more soluble 

 salts were precipitated. These are represented by the Stassfurt 

 deposits, which probably extend far to the west of their present 

 proved position, or by deposits of similar nature now denuded. 



The field evidence is in favour of some of the dolomite being 

 originally deposited as a precipitate. Thick beds of powdery dolomite 

 (some of which are called "marl") 1 occur in a very fine state of 

 subdivision which would appear to have come down as dolomite, 

 otherwise it would be expected that in the slow process of formation 

 by the association of the carbonates after deposition, crystallization 

 would have taken place in these rocks, and rhombs of dolomite have 

 been formed. The dolomitic oolites also appear to me, 2 as Trechmann 

 has already asserted, to support the view that the dolomite was 

 originally deposited round the centre of the oolith. On the other 

 hand, some of the more coarsely granular dolomitic rocks composed of 

 coarse dolomite rhombs 3 may have been formed by the slow 

 crystallization of the dolomite from the association of the free 

 carbonates after precipitation. It may thus be that in the Durham 

 Permian dolomite has sometimes been precipitated, while at other 

 times the two free carbonates have been laid down to subsequently 

 combine into that mineral. 



It has already been stated that the beds of magnesian limestone 

 differed considerably in composition at the time of their deposition, 

 but since then important changes have taken place on various- 

 horizons. Dolomitization of certain parts has been brought about 

 either pene-contemporaneously with the deposition of the strata or 

 long after they were formed, 4 portions have been silicified, 5 other 



1 Trechmann gives an analysis of the Great Marl bed of Fulwell Quarries — 

 Dolomite 78-42 



Calcite 



Insoluble residue 

 Calcium sulphate 

 Iron or manganese 



13-04 

 5-02 



trace 

 2-41 



2 Trechmann gives an analysis of dolomitic oolite from Haswell — 



Dolomite 96-49 



Calcite 3-80 



Anhydrite gypsum or sulphur trioxide . 0-07 

 Iron or manganese . . . . . 0-57 

 Insoluble residues ..... 0-02 



3 The coarse granular dolomite occurring in the Cotefield Close boring gives 

 (analysis by A. D. N. Bain, B.Sc.)— 



Dolomite 62-38 



Calcite 30-10 



Ferric oxide . . . . ... 3-88 



Insoluble residue ..... 3-46 



4 [For note see next page.] 



5 Silicified limestones occur at the south end oJ Marsden Bay, Hendon, etc., 

 and Trechmann has noticed silicified oolitic limestones. 



