FOSSILS, AND HOW THEY ARE FORMED 11 



filtering in of water charged with Ume or sih- 

 ca, which is there deposited, often in crystal- 

 Hne form. In this way, too, are formed casts 

 of eggs of reptiles and birds, so perfect that it 

 is possible to form a pretty accurate opinion 

 as to the group to which they belong. 



Sometimes it happens that shells or other 

 small objects imbedded in hmestone have been 

 dissolved and replaced by silica, and in such 

 cases it is possible to eat away the enveloping 

 rock with acid and leave the silicified easts. 

 By this method specimens of shells, corals, 

 and bryozoans are obtained of almost lace-like 

 delicacy, and as perfect as if only yesterday 

 gathered at the sea-shore. Casts of the interior 

 of shells, showing many details of structure, 

 are common, and anyone who has seen clams 

 dug will understand how they are formed 

 by the entrance of mud into the empty shell. 



Casts of the kernels of nuts are formed in 

 much the same way, and Professor E. H. Bar- 

 bour has thus described the probable manner 

 in which this was done. When the nuts were 

 dropped into the water of the ancient lake the 

 kernel rotted away, but the shell, being tough 



