154 Wisconsin Academy of Sciences, AriSj and Letters. 



from the under surface are always formed during a rising tem- 

 perature, and are in respect to form similar to those which form 

 from the upper surface downw^ard during a period of falling 

 temperatures. 



When cracks form from the lower surface the unfrozen water 

 soon fills the spaces thus formed. If the temperature is still 

 one of freezing or if it soon changes from thawing to freezing, 

 this water will solidify, making the ice cap solid again from 

 bank to bank. 



In general, all cracks, whether they form from the upper or 

 under surface, may be divided into two classes. The first con- 

 sists of those that penetrate the entire thickness of the ice and 

 the second comprises those that penetrate only part way. The 

 cracks of the first class often extend in straight lines over long 

 distances. Those of the second class are mainly short and dis- 

 continuous, and often form a veritable net work. The cracks 

 of the first class are sho^vn in Plate XV, and those of the second 

 class in Plate XIV. 



The stresses on the ice sheet are very complex and the fissures 

 which result extend in many directions and are indefinite in 

 length. The fractures are frequently very complex, resembling 

 the spiral or feather structure shown in Plate XVI. Short 

 cracks, not over two or three inches long, were frequently ob- 

 served crossing the larger fractures at right angles. Where an 

 especially prominent fracture has formed it is common to find 

 several less important ones near by and parallel to it. 



A number of cracks were observed in which the walls at the 

 surface were one-fourth inch apart, and yet which apparently 

 had not been filled with water from below, the ice being about 

 fourteen inches thick. Most cracks at the surface of less width 

 than this, however, w^ere filled with ice. 



The theory that contraction is a maximum at the upper sur- 

 face and a minimum at the under surface leads to the assump- 

 tion that the ice filling a crack which forms from the upper sur- 

 face will have the shape of a wedge as shown in Plate XVII. 

 The ice filling a crack which forms from the under surface will 

 likewise have the same shape, except that the head will be down 

 instead of up. 



An examination of a considerable number of ice cracks dur- 



