12 ANIMALS OF THE PAST 



and hard, would probably last for years un- 

 der 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 in- 

 terior of the nut. At the same time with this 

 process, carrying lime carbonate in solution 

 was going on, so that doubtless the stone ker- 

 nels, 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 of weathering. Not so the or- 

 ganic shell ; this eventually would decay away, 

 and so leave the filling or kernel of chalcedony 

 and lime.* 



" Fossil leaves " are nothing but fine casts, 

 made in natural moulds, and all have seen 

 the first stages in their formation as they 



* Right here is the weak spot in Professor Barbour's ex- 

 planation, and an illustration of our lack of knowledge. For 

 it is difficult to see why the more enduring husk should not have 

 become mineralized equally with the cavity ivithin. 



