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

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

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

 slow natural sublimation into the atmosphere, leaving open cracks 

 in the mud. In the locality where this cast was made, the crystals 

 were relatively slender, distant, and some gently curved, like those 

 seen in Plate V. Slightly reduced. 



