GEOLOGICAL AND NATURAL HISTORY SURVEY. 81 



Dr. Andrews estimates that the combined bulk of the beaches 

 formed by Lake Chicago is nearly equal to that of the beach of the 

 present lake, the proportion being i6 to 17. In this computation 

 it was assumed that 656,000,000 cubic yards escaped through the 

 Chicago outlet. This assumption is based on a comparison of the 

 relative sizes of the beaches of Lake Chicago in a section outside 

 the outlet and a section embracing the outlet. 



Dr. Andrews attempted to estimate the length of time in- 

 volved in the accumulation of the beach deposits by measuring the 

 amount of sand carried southward past the piers at Chicago and 

 Michigan City. The sand annually stopped by the two piers was 

 found to be 129,000 cubic yards. If this represented the whole 

 drift past the piers the period represented in the accumulation of 

 the sand in all the beaches would be 26,000 years, and the duration 

 of Lake Michigan at its present stage 13,000 years. He estimates, 

 however, that not more than one-fourth or one-fifth of the south- 

 ward drifting sand is stopped by the piers, and thus reduces the 

 period to less than 6,000 years, with but about 3,000 years for Lake 

 Michigan. 



Dr. Andrews' estimates were based on the assumption that 

 tliere is a southward flowing current on each side of the lake, car- 

 rying sand to its present head. Investigations made by the 

 Weather Bureau in 1892 and 1893, under the. direction of Prof. 

 Mark Harrington (i) led him to the conclusion that the currents 

 ov, the east shore in the southern portion of the basin are north- 

 ward instead of southward. He accounts for the accumulation of 

 sand on the north side of breakwaters along this coast by the action 

 of the surf, in northerly storms, which is more transient than the 

 currents proper and would affect the southern part of Lake Michi- 

 gan only when the wind was northerly. This occasional phe- 

 nomenon is very efificient when it occurs. He concludes that the 

 estimates of time involved in the formation of beaches have, there- 

 fore, less value than would be the case were the accumulations due 

 more largely to lake currents. 



Considerable study of the movement of water in Lake Michi- 

 gan has been made by the Chicago Drainage Commission, largely 

 under the direction of Professor Cooley. As a result of these in- 

 vestigations, which involve not only a study of bottle papers, but 

 also a thorough canvass of the opinions of lake captains and an 

 examination of breakwaters, Mr. Cooley has reached the conclu- 



(1) Currents of the Great Lakes as deduced from the movemerxts of Bottle 

 Bapers during the seasons of 1892 and 1893, by Mark W. Harrington, Weather Bureau, 

 Bulletin B, U S. Department of Agriculture, 1894, 



