FOSSILS, AND HOW THEY ARE FORMED 11 
filtering in of water charged with lime or sili- 
ca, which is there deposited, often in crystal- 
line 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 limestone 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 casts. 
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 
