202 



such a system of currents as is 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 contour of the shores. If 

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

 months, the surface water api>ears to have a southward movement through- 

 out 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 iialf of the lake. As the prevailing 

 winds are often from the west, these become the most protracted of the 

 movements of the surface water. Cooley has found that breakwaters 

 along the shore sui)port this interpretation. In the southern half of the 

 lake they are largely constructed to protect tJie harbors from the drift 

 on the north side, while in the northern half they are constructed to 

 protect them from drift coming from the south. In view of this ap- 

 parently changeable course of lake movements, it seems doubtful if esti- 

 mates, such as Dr. Andrews attempted, have the value that some have 

 attached to them.' 



"Near the shoi'e the bottom of Lake Michigan is uniformly covered 

 with .sand. At the slioro line this sand is about ten feet deep and it ex- 

 tends out to where tlie wati'r reaches a depth of about thirty-five feet. 

 Beyond this depth of water the lake bottom is composed of a stiff, tena- 

 cious blue clay, wliicli is said to contain partings, or pockets of sand from 

 wlience, in [tart, conies the supply which is constantly being carried to the 

 shore by the waves. Much of this sand is doutbtless blown fr mi the dunes 

 by south wir.ds back over the lake, and. falling on its surface is again 

 brought to land. Moreover, by storms and by ice jams in the spring all 

 projecting points along the lake are slowly wnni down and the material 

 composing tliein is c.-irried out to be again returned and built up in a new 

 place. Thus much of the sand is in constant circulation, and the necessary 

 new supply is not as great as it seems to be. 



".Much gravel, consisting of pebbles ranging in size from the size of a 

 lion's egg to that of a small marble, is washed uj) by the waves to within 

 a foot or two of tlie margin of the water. In ninny places it is raked out 



> U. S. G. S. Monograph XXXVIII, pp. 4.5.5-.''»0. 



