Yol. 59.] CELLULAR MAGNESIAS LIMESTONE OF DURHAM. 51 



5. The Cellular Magnesian Limestone of Durham. By George 

 Abbott, Esq., M.R.C.S., E.G.S. (Read December 3rd, 1902.) 



[Abstract.] 



The Permian Limestone covers about 1J square miles near Sun- 

 derland ; it alternates with beds of marl containing concretionary 

 limestone-balls, and attains a thickness of 65 feet or so. The 

 cellular limestones frequently contain more than 97 per cent, of 

 calcium-carbonate. Magnesium-carbonate occupies the interspaces 

 or ' cells ' of this limestone, and also the spaces between the balls. 

 The hundred or more patterns met with in it can be arranged into 

 two chief classes, conveniently termed honeycomb and coralloid, 

 each with two varieties ; and each class has four distinct stages, 

 both classes having begun with either parallel or divergent systems 

 of rods. The second stage is the development of nodes at regular 

 distances on neighbouring rods ; and these in the third stage, by 

 lateral growth, become bands. Finally, in the fourth stage the 

 interspaces become filled up. The upper beds are usually the most 

 nearly solid. In the coralloid class the nodes and bands are smaller 

 and more numerous than in the honeycomb class. In both classes 

 tubes are frequently formed. The rods have generally grown down- 

 wards, but upward and lateral growth is common. A section of 

 Fulwell Quarry is given.. 



Discussion. 



Dr. Henry Woodward complimented the Author, not only upon 

 his fine display of lantern-slides and photographs, but also upon 

 having liberally presented to the Natural History Museum the very 

 beautiful series of specimens which he had collected during many 

 years. The Author had referred to the 'puzzle/ which, since the 

 days of Sedgwick, still remained, as to how these structures came 

 about. Surely, the giving of names to the varied forms which 

 these remarkable inorganic bodies took on, did not advance us 

 much. He (the speaker) thought that we should look at them, 

 and at the flints in the Chalk, the clay-ironstone nodules of the 

 Coal-Measures, the septaria of the London Clay, and the con- 

 cretions in other clayey and shaly beds, as all due to the same set 

 of causes. Water in the Chalk, charged with silica in solution, 

 deposited that silica as flint-nodules or bands of flint along lines 

 of stratification in the Chalk or in joints. So in other beds the 

 iron was deposited, often around organisms ; but not so much so 

 at Sunderland, although Prof. Garwood had shown that fossil shells 

 did occur in these calcareous concretionary beds. Prof. Eainey, as 

 far back as 1857, had pointed out that, by introducing gum in solution 

 into a fluid magma ready to crystallize out, the tendency to crystallize 

 remained, but was frustrated or arrested by the gum-solution, and 



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