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



after chalybite) also occur. Parts of the limestone contain a small 

 amount of carbonaceous matter of organic origin, and some of the 

 limestones are fetid. 



The limestones vary in colour, texture, macroscopic and microscopic 

 structure considerably. These variations are partly due to the 

 differences in composition and partly to the different methods of 

 formation of the beds, but also largely to the nature of the physical, 

 chemical, and structural changes that have taken place in them. In 

 colour they range through dark and light brown, yellow, grey to pure 

 "white rocks, and the numerous varieties of their texture include fine, 

 soft and powdery, hard and compact, close-textured, porous, coarsely 

 cellular, concretionary, crystalline, segregated, brecciated, pseudo- 

 brecciated, etc. Thick beds of granular dolomite occur, and in 

 the Middle and Upper divisions strata of dolomitic oolite reach 

 a considerable thickness. Parts of the limestone consist of the 

 remains of organisms with a dolomitic or calcareous matrix. In 

 microscopic texture they vary from rocks showing little or no 

 structure to beds built up of allotriomorphic grains and idiomorphic 

 rhombs of dolomite and crystalline gi'ains of calcite. In some parts 

 of the limestone crystallization has gone on to such an extent (quite 

 apart from the highly altered concretionary, segregated, and other 

 limestones where the change is evident in the field) as to entirely 

 alter the internal structure. Original oolitic rocks have been altered 

 so as to leave but little trace of their structure, and from parts of the 

 Bryozoa Peef the fossils have been entirely obliterated. 



Parts of this limestone are of organic origin, this being notably the 

 casein the shell bank of the Bryozoa Reef, which consists of remains 

 of invertebrate life that flourished under the protection of the 

 Bryozoa, together with an infilling of calcareous or dolomitic sediment. 

 The pure calcareous inorganically formed rocks, and some of the 

 dolomitic limestones are the result of sedimentation. The oolites of 

 the magnesian limestones, which attain a considerable thickness 

 (100 feet) in parts of the Middle and Upper limestones, are specially 

 interesting. The concentrically arranged material of the ooliths is 

 dolomite with centres of calcite, gypsum, or fluorite. 1 Gypsiferous 

 oolites have been proved in the borings in South Durham, and the 

 hollow oolites of Poker and Hartlepool had originally gypsiferous 

 centres. The fluorite is probably secondary, being deposited after 

 the solution of the gypsum. These dolomitic and gypsiferous 

 dolomitic oolites were produced by the deposition of material round 

 crystalline grains or nuclei either as they fell through the water or 

 lay on the bottom of the sea. 2 They show no trace of current 

 bedding, 3 were probably laid down in fairly deep water, and do 

 not appear to have been produced through movement by current 

 action, nor were they formed by organic agency. 



The dolomitic limestones when the sulphates have been removed 



1 Trechmann, Q.J.G.S., vol. lxx, p. 250. 



2 " On Oolites and Spherulites " : H. Bucher, Joum. of GeoL, vol. xxvi, 

 No. 7, Oct.-Nov., 1918. 



3 Current bedding is traceable locally in the Roker oolite, and I have noticed 

 it slightly developed in an altered oolite in Silkworth Quarry. 



