82 CHICAGO ACADEMY OF SCIENCES. 



sion that the effective work on the shores is due to waves and not 

 to currents, and it is a matter of doubt if this lake has such a system 

 of currents as are indicated by Professor Harrington's charts. The 

 movement of the water seems to depend mainly upon the wind, but 

 is governed to some degree by the contours of the shores. If the 

 north winds prevail for a few days, as is often the case in the spring 

 months, the surface water appears to have a southward move- 

 ment throughout the breadth of the lake, and return currents must 

 be at some depth. On the other hand, a prevailing south wind, 

 such as occurs for short periods during the summer, will induce 

 a northward movement across the entire breadth of the lake. The 

 contours of the shore seem to favor a northward movement from 

 direct west winds in the north half and a southward movement in 

 the south half of the lake. As the prevailing winds are from the 

 west these become the most protracted of the movements of sur- 

 face water. Professor Cooley has found that breakwaters along 

 the shore support this interpretation. In the southern half of the 

 lake they are largely constructed to protect the harbors from the 

 drift on the north side, while in the northern half they are con- 

 structed to protect them from drift coming from the south. In 

 view of this apparently changeable course of lake movement, it 

 seems doubtful if estimates, such as Dr. Andrews attempted, have 

 the value that some have attached to them. 



Dr. Andrews also made an estimate of the age of the lake from 

 the annual amount of destruction of the bluffs. To determine the 

 rate of erosion on the west coast of Lake Michigan he gathered a 

 large number of observations, mostly derived from surveys, and 

 after rejecting loose or vague estimates, as well as erosions 

 brought to notice because of remarkable rapidity, he obtained the 

 results given in the following table : 



At Evanston the erosion is ••....• • 16.95 feet a year 



At the Old Pier, two miles further north 4.90 " " 



One mile further north • 3.08 



At Winnetka • ■ • • ■ • 4.05 



One mile further north. ■ .......•■ 6.05 



Lake Forest •• ••• 1.65 " " 



Waukegan 0.00 



Two miles further north. . 0.00 



State line • 16.50 



Kenosha --... 12.00 



Two miles further north. • . . - • ■ • 3.00 " 



Three miles further north ...••.... 12.00 " ' * 



