CASTS OF SHELLS. 257 



as the animal matter or gelatine decayed, water containing 

 some dissolved mineral has filtered in and filled up all the 

 interstices, it may be with silica, or it may be with some 

 metal, and the shell is thus more or less mineralised. 



Very often when the mud in which a shell has been 

 buried has become hard, the shell itself has been dissolved 

 away, and all that remains of it is a cast of the interior in 

 hardened mud or stone, and an impression of the exterior, 

 with an empty space between the two. This empty space 

 is again often filled with mineral matter, so that we have a 

 perfect cast of the whole shell inside and out. 



The beds known as greensands are, it is said, largely 

 composed of the minute internal casts of foraminifera 

 (Fig. 28), whose tiny shells, before they dissolved away, were 

 filled with silicate of iron and potash. 



About two-thirds of a bone consists of the earthy matter 

 (phosphate, carbonate, &c.) already mentioned ; the re- 

 maining third is animal matter, a sort of gelatine, and as 

 this decays, mineral matter may filter in, and the bone 

 become petrified or mineralised. Both animal and vege- 

 table substances may be mineralised to a certain extent in a 

 few weeks, or even days, when steeped in mineral water. 



Occidental turquoise seems to be nothing more than 

 fossil bone or ivory, coloured by the infiltration of phosphate 

 of iron, while the true turquoise, which it resembles, but 

 does not equal, consists of phosphate of alumina coloured 

 by copper. 



But there is another and more wonderful method of 

 petrifaction which is by no means uncommon, to which we 

 have alluded in Chapter XII. In this the shell, bone, or tree 



