Plate X. Photograph of a cast made by pouring plaster over a 

 surface of mud in which ice crystals had recently formed, in Rock 

 Island, 111., and where they had later been removed by natural 

 sublimations into the atmosphere, leaving open cracks in the mud. 

 It will be seen that the crystal growth in this case involves an x-like 

 or radiating pattern formed of relatively very slender forms that 

 almost everywhere are very gently curved somewhat reminding of 

 the slender thread-like crystals sometimes seen in frost on windows. 

 I have not yet seen any similar fossil crystal growths as slender as 

 these. Slightly reduced. 



