96 PROCEEDINGS OF SOCIETIES. 



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 fresli-water 

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

 being composed of such particles as result from tlie decay of Lim- 

 naeans, in which term are included LimncBUs and Planorbis. 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 Charge ; but they have undergone more or less of crystalline 

 consolidation. As examples of them, two physical analyses may be 

 given of specimens 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 Limnseans : 

 Empty cavities . . . . . 16'3 



Fragments of Limngeans . . . . 15"6 



Fragments of CAar-a . . . . 11"0 



Fine grains of decayed Limnseans and Chara 57*0 

 Peroxide of iron . . . . . "1 



100-0 

 2. A hard, even-grained specimen, with no entire or large frag- 

 ments of shells visible to the naked eye : 



Grains of Limnsean shell showing structure ^ • 7 1 , „ . /% 

 Ditto not showing ditto . . . 12'3| 



Crystallized fine granules of shell, &c. . 55*9 



Quartz sand . . . . . 13 '5 



Very fine sand and decomposed felspar . 12" 1 



Peroxide of iron, chiefly in the substance 



of shell fragments .... '5 



100-0 

 In the above-described marls and limestones are found several 

 curious bodies, but in no great proportion ; and, on the whole, they 

 may be said to be derived from the decay of the fresh-water shells 

 found in them, and not from the deposition of chalky mud, which 

 has a totally different character, though the calcareous matter in the 

 water, from which the shells procured it, may have been derived 

 from the contiguous chalk. It is worthy of remark, that in these 

 marls no Diatomacem are found, though they abound in the clays 

 associated with some of them ; but the examination of tufaceous 

 travertins has furnished the author with evidence which proves that 

 contact for a long period with carbonate of lime decomposes rnd 

 destroys their siliceous coverings, and therefore they could hardly 

 be expected to occur in such deposits as those under consideration. 



