SECONDARY LIMESTONES. 239 



The Magnesian limestone, as the next in order, sue- 

 ceeds to the coal strata when these are present; while, 

 in other cases, following the supposed transition class, 

 it hecame the lowest member of the secondary series, 

 or the first flcetz limestone of the German school, as 

 already noticed. The presence of conglomerate beds 

 consisting of fragments of the inferior limestone, is one 

 of the most interesting features of this deposit ; con- 

 firming the former remarks respecting the production 

 of mixed rocks at the intervals of revolution among 

 the strata. Thus the conglomerate, like the mountain 

 limestone itself, is also occasionally cavernous. 



The strata are of variable thickness, sometimes con- 

 taining the usual associated substances; and occasion- 

 ally including the gypsum and salt which belong, more 

 properly, to the sandstone above it. The presence of 

 magnesia, sometimes amounting to twenty per cent., 

 is one of its most remarkable characters, though far 

 from a constant one ; while, as it occurs also in the 

 mountain limestone, these deposits have occasionally 

 been mistaken for each other. It is generally of a 

 granular or sandy texture, becoming sometimes ooli- 

 thic, and also cellular, while often, further, possessing 

 a slight lustre ; and the colours are commonly buff, 

 passing into brownish and reddish tints ; more rarely, 

 white. It sometimes contains lead and iron. Organic 

 remains are rare in this limestone; but fishes have oc- 

 curred, together with entrochi, encrinites, flustroe, 

 donax, area, and anornia. If a Monitor has been 

 found in " marl-slate" said to belong it, we may be 

 allowed to doubt the accuracy of the observation. 

 Eight fuci are named as having also occurred; adding 

 however but a superfluous proof of that marine origin 

 which every thing else indicates. 



The Muschelkalk of the Germans is esteemed by 



