FOSSILS, AND HOW THEY ARE FORMED 7 



had changed to stone, and the filling up of this space by 

 the filtering in of water charged with lime or silica, 

 which is there deposited, often in crystalline 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 silici- 

 fied 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. Barbour has thus de- 

 scribed the probable manner in which this was done. 

 When the nuts were dropped into the water of the an- 

 cient lake the kernel rotted away, but the shell, being 

 tough and hard, would probably last for years under 

 favorable circumstances. Throughout the marls and 

 clays of the Bad Lands (of South Dakota) there is a 

 large amount of potash. This is dissolved by water, 

 and then acts upon quartz, carrying it away in solution. 

 This would find its way by infiltration into the interior 

 of the nut. At the same time with this process, carry- 

 ing lime carbonate in solution was going on, so that 

 doubtless the stone kernels, consisting of pretty nearly 

 equal parts of lime and silica, were deposited within 

 the nuts. These kernels, of course, became hard and 

 flinty in time, and capable of resisting almost any amount 



