1853.] SORBY STRUCTURE OF LIMESTONES. 345 



The particular forms of the particles into which the Limnssans 

 and Paludinse, found so plentifully in many freshwater marls, are re- 

 solved by decay, were then described and shown to present such de- 

 finite characters as to render it easy to distinguish them with certainty 

 from most others at all likely to occur in them. 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 with 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 miich 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 these 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 

 numerous opercula of the Bithinia occur, which therefore appear to 

 have been less prone to decay than the shells themselves. Other 

 similar marls of post-tertiary age were also described, and shown to 

 have resulted from the decay of similar shells in variable proportion. 



The soft marly portions of the Isle of Wight tertiary freshwater 

 limestone were stated to be of precisely the same nature as the above, 

 being composedof such particles as result from the decay of Limnseans, 

 in which term are included Limnceus and Planorhis. The examination 

 of thin sections of the harder varieties of the same limestone also 

 shows that they were derived from the same source, mixed with a 

 variable, sometimes very large proportion of fragments of Charae ; 

 but they have undergone more or less of crystalline consolidation. 

 As examples of them, two physical analyses may be given of speci- 

 mens from Binsted, which will also serve to show the character of 

 such analyses. 



1 . A hard, marly-looking specimen, with numerous cavities due to 

 the removal of the shelly matter of more or less entire LimuEeans. 



Empty cavities 1()'3 



Fragments of Limnseans 15*6 



Fragments of Chara • •• . 1 1 '0 



Fine gi'ains of decayed Liranaeans and Chara. . 57'0 



Peroxide of iron '1 



100-0 



