486 E. M. KINDLE SEPARATION OF SALT FROM SALINE WATER 



Another variety of surface configuration which is occasionally seen on 

 the Salt Kiver salt plains is an irregular flat-topped elevation usually 6 

 or 8 inches in diameter, which rises only an inch or two above the surface 

 of the mud-crack polygons on which it occurs. These closely resemble in 

 their irregular outline, slight elevation and association with mud-crack, 

 the Pamelia limestone mud-crack hummocks described above. Some spe- 

 cies of algae probably acts as a nucleus for the development of these 

 biscuit-shaped masses. 



Large areas of the mud-crack polygons of the Salt Eiver salt plains 

 have a deeply corrugated surface (figure 10). This peculiar surface is 



Figure 11. — Desiccated saline Clay with dried Algce, Salt River, Northwest Territory 



Half natural size 



due to a species of algae which evidently grows over large areas of the 

 plain when it is covered by water. After the drying up of the playa the 

 shrinkage of the algae, combined with the irregular efflorescence of the 

 subsurface salt and saline mud which it develops, results in the innumer- 

 able closely spaced zigzag ridges one-fourth to one-half an inch in height 

 (figure 11). 



In the Salt River playas two types of mud-crack are common. In one 

 the polygons are marked by lines which are distinct and sharply defined, 

 but which, as in the Kingston mud-crack, scarcely cut below the surface. 



