7ol. 57.] SEAM OF COAL IXTO A. SEAM OP DOLOMITE. 305 



almost total disappearance of clastic material of vegetable origin 

 within the space of 400 or 500 yards? It was not absolutely 

 necessary to olfer an alternative view. The objections to any theory 

 involving the replacement of coal by dolomite were very great, 

 while the question itself was essentially one for chemists. Never- 

 theless, an explanation in this direction should not be regarded as 

 impossible : the peculiar reddening of the measures above the 

 dolomitic stone was worthy of attention in this connection. 



Dr. CuLLis said that the mode of origin which the Author had 

 suggested for this peculiar band of dolomite seemed to him to be a 

 very likely one, for it appeared to explain the facts in a satisfactory 

 manner. Bearing in mind the probable nature of the waters in 

 which the Coal-Measures accumulated, their direct derivation from 

 the land, their shallowness, and the constant and rapid evaporation 

 to which they were doubtless subjected, it did not seem improbable 

 that occasionally the carbonates of calcium and magnesium, which 

 they carried in solution, might be precipitated in the form of 

 magnesian limestones or of dolomites. There seemed to be at least 

 two distinct methods in which sedimentary dolomites (as distinguished 

 from crystalline dolomites which have acquired their crystalline 

 texture as the result of metamorphism) had originated : firstly b)' 

 contemporaneous precipitation from solution, and secondly by the 

 substitution of magnesium for a part of the calcium in a normal 

 limestone by a process of subsequent replacement. The rock 

 described by the Author, and also the Permian Magnesian Limestone 

 (which exhibited structures not at all unhke those displayed by the 

 dolomite-band under discussion), might be examples of rocks origin- 

 ating essentially by the first of these processes ; while the dolomitic 

 parts of the Carboniferous Limestone or of recent coral-reefs might 

 be instanced as examples produced by the second. Students of 

 dolomites were indebted to the Author for having recorded this 

 occurrence of a rock which, though apparently rare, perhaps unique, 

 in British deposits, was nevertheless one which from a priori con- 

 siderations might very reasonably have been expected to occur. 



Mr. Walcot Gibson stated that the spherulitic siderite referred to 

 by the Author occurred in large quantities in I^orth Staffordshire, 

 either aggregated into bands and nodules, or disseminated throughout 

 the marls and clays of the upper portion of the Middle Coal-Measures 

 and of the lower part of the so-called Upper Coal-Measures. In the 

 latter, the well-known Black-Band Ironstones were developed : these 

 contained a considerable amount of vegetable and organic matter. 

 It was a recognized fact that the Black-Band Ironstones invariably 

 overlay coals, the ironstone varying in thickness in inverse ratio to the 

 thickness of the coal. In the western part of the Pottery Coalfield, in 

 the Minnie shafts, the ironstones appeared to be replaced in part by 

 limestones. It was extremely likely that the phenomena described 

 by the Author were present in the upper part of the Coal-Measures 

 in North Stafi'ordshire, and that rocks of the type described in the 

 paper were formed over large areas in the Midlands during the 

 closing stages of the Coal-Measure Period. 



Mr. it. D. Oldham considered that the paper was one of great 



