32 N. M. FENNEMAN 
necessarily powerful undertow. In Lake Mendota, where wave- 
base is not lower than twenty feet, sands and even heavy gravels 
are irregularly distributed over the bottom at depths frequently 
approaching fifty feet. Some lie at the bases of steep slopes 
which gravity may have helped them to descend, but others are 
far from slopes and plainly illustrate the erosive power of cur- 
rents resulting from a concentration of movement along certain 
lines. N. M. FENNEMAN. 
THE UNIVERSITY OF CHICAGO. 
