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PROCEEDINGS OF SOCIETIES. 



GEOiiOGicAii Society. 



On the Microscopical Structure of Freslavater Marls and Lime- 

 stones. By H. Clifton Sorbey, F.G.S. 



The author first described the general conclusions he had arrived at 

 with respect to the condition of the mineral portion of calcareous 

 organisms, which he considered is first deposited in the form of 

 crystalline granules of variable size, that afterwards undergo more or 

 less of crystalline coalescence. In some cases this scarcely occurs at 

 all ; but in others it does to a very considerable extent during the life 

 of the organism, and this produces a great difference in tiie character 

 of the particles into which it is resolved by decay, Tlie falling to 

 powder that then takes place is the result of the oxidization and 

 removal of the organic portion, and, if no crystalline coalescence 

 had occurred, the shell or other body might be resolved into the very 

 minute, ultimate, crystalline granules ; whereas, if much coalescence 

 had taken place, it would break up into much larger ones, showing 

 in many instances its minute organic structure. 



The particular forms of the particles into which the Limnseans 

 and Paludinse, found so plentifully in many fresh-water marls, are 

 resolved by decay, were then described and shown to present such 

 definite characters as to render it easy to distinguish them with cer- 

 tainty from most others at all likely to occur in ihem. Soft, loose 

 marls can of course be investigated by mixing the particles in water; 

 but thin sections of harder limestones must be prepared, and the facts 

 which may be learned from them are in many respects very superior ; 

 and from them the relative proportion of the various constituents 

 may be determined with great accuracy, by carefully drawing their 

 outline on strong even paper witii a camera lucida, and afterwards 

 cutting out the several portions and weighing them. This method 

 the author terms " physical analysis." To fully describe all the 

 necessary particulars would occupy too much space for this abstract ; 

 but, by attending to them, very great accuracy may be attained, and 

 the true physical constitution of the specimen stated in a manner 

 quite different from what could be ascertained by chemical analysis, 

 which, for the purpose of these inquiries, is often greatly inferior, 

 though often most valuable in addition. 



Proceeding to the application of tliese methods of research to 

 particular cases, some white marly deposits found in some of 

 the filled-up lakes of Holderness were described, and shown to be 

 composed of such particles as result from the decay of Bithinia 

 tentaculata, mixed with a small but variable proportion of such as 

 are derived from decayed Limnseans. In confirmation of this it 

 may be stated, that though no entire shells are found in them, yet 



