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

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

 Island, Illinois, after the ice in the crystals had been removed by 

 slow natural sublimation into the atmosphere, leaving open cracks 

 in the mud. The comb-like ridges on the plaster cast have the form 

 of the ice crystals. Compare with Plate III. 



