THE 



GEOLOGICAL MAGAZINE. 



NEW SERIES. DECADE Ml. VOL. VIII. 



No. X.— OCTOBER, 1891. 



oiax<3-iisr.A.Xi JLiaTioXiES. 



I. — On the Origin and Mode of Formation of the Concretions 

 IN the Magnesian Limestones op Durham. 



ByE, J. Garwood, B.A.,.F.G.S. 

 (PLATES XII. and XIII.) 



IN 1826, 27, and 28, Prof. Sedgwick contributed to the Geological 

 Society of London his classical papers on the Magnesian 

 Limestone of the East of England,^ in which he describes in detail 

 the concretionary structures which occur in many of the beds of this 

 formation in Durham. 



Although he describes these concretions in minute detail, he says 

 but little as to their probable origin, but it is clear that he regarded 

 them as composed of carbonate of lime derived from the beds in 

 which they occur, when he says — "The particles (of lime), after 

 deposition, appear to have run into lumps and masses more or less 

 crystalline, rejecting great portions of the earthy residuum." ^ Since 

 Prof. Sedgwick's papers little seems to have been contributed to the 

 subject of the mode of formation of these concretions. 



Two opposite opinions are held with regard to the proximate 

 source of the carbonate of lime : — One to the effect that the lime 

 was introduced into the concretionary bed subsequent to its forma- 

 tion, in solution from without ; the other that it was deposited 

 contemporaneously and subsequently segregated out in situ. 



The latter opinion is, we believe, that genei'ally held,^ though 

 there are some who advocate the former, which may be called "the 

 Stalactitic " * theory. 



Looking at the Magnesian Limestone series as a whole, many of 

 the concretionary structures appear as if they might have been formed 

 in the stalactitic manner supposed ; but when the individual beds 

 containing the concretions are studied in detail, they are found to 

 present many points which appear irreconcilable with the theory of 

 their stalactitic origin. 



The locality where these beds can be most satisfactorily examined 



^ Prof. Sedgwick, " On the Geological Eelations and Internal Structure of the 

 Magnesian Limestone," Trans. Geol. Soc. London, 2nd series, Yol. iii. 1835. 



~ Op. cit. p. 89. 



2 Prof. Green, Physical Geology, p. 279 ; Geikie, Textbook of Geology, 1885, 

 p. 472. 



* R. Howse, " On the Stalactitic Origin of Conglobated Structures in the 

 Magnesian Limestone," Tyneside Naturalists' Field-club, October, 1889. 



decade III. — VOL. Till. — NO. X. 28 



