in the Lower Greensand of Bedfordshire. 383 



The conglomerate of the Lower Greensand of Bedfordshire con- 

 sists of ferruginous sand more or less indurated (which does not 

 effervesce with hydrochloric acid), rolled pebbles, and light-brown 

 nodules of phosphatic matter, which have an earthy fracture and 

 often contain fragments of shells*. The nodules are often co- 

 vered with perforations, which Mr. A. Wanklyn discovered to be 

 the work of small bivalves f. Concretions of peroxide of iron 

 are also found in this deposit. 



The fossil shells found in this bed exist in two different con- 

 ditions, some being casts composed of the same material as the 

 nodules, whilst the shells of others are replaced by oxide of 

 iron, and are filled with the same material of which the bed is 

 composed ; the indurated part of the bed also contains numerous 

 impressions of shells. It is difficult to see how two formations 

 presenting such marked points of difference can have been de- 

 posited under the same conditions. 



The remains of organized bodies contained in this bed, as I 

 have before stated, exist in two different states of mineralization, 

 viz. as ferruginous shells and as phosphatic casts. The bed being 

 very porous (a well has been sunk 50 feet deep for water) and 

 largely impregnated with ferric oxide, shells (which, as is well 

 known, consist chiefly of animal matter and calcic carbonate) 

 would by the action of water have their calcic carbonate replaced 

 by ferric oxide. In cases where the action was more rapid, only 

 internal casts of the shells would remain. 



Shells which lived in the sea whilst this bed was being formed^ 

 and also shells derived from older formations, if deposited in 

 this bed, would undergo this change. 



The phosphatic casts of shells must have been formed, or, at 

 least, the shells must have been filled with phosphatic matter, be- 

 fore they were deposited here, and the calcic carbonate afterwards 

 dissolved by the action of water. The tricalcic phosphate would 

 be protected from the solvent action of water by the presence of 

 calcic carbonate, as proved by the experiments of Mr. R. War- 

 rington, junr., described in a paper read to the Chemical Society. 

 The ferruginous shells and phosphatic casts are found inter- 

 mixed. 



The conglomerate contains lumps of hardened clay; and the 

 so-called coprolites contain a much larger percentage of alumina 

 than those of the Cambridge Greensand. The analysis of the 

 coprolites is made from a commercial sample, which contains the 

 shells, nodules, teeth, and bones, all ground up together. The 



* In the interior of the nodules there are sometimes found specimens 

 of a small species of Lima and of Cardium. 



t Mr. Wanklyn has obtained several of these shells, which appear to be- 

 long to two or three different species. 



