202 



sufli a system of cuneiits as is indicated by Professor Harrington's 

 cluirts. The movement of the water seems to depend mainly upon tlie 

 wind, hut is governed to some degree by the contour of the shores. If 

 the north winds prevail for a few daj's, as is often the case in the spring 

 months, the surface water appears to have a southward movement through- 

 out the breadth of the lal^e, and return currents must he 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. I'he contours of the shore seem to favor 

 a northward uiovenicnt 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 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 constructed to 

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

 parently changeable course of Take nioTements, it seems doubtful if esti- 

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

 nttached to them.^ 



"Near the shore the bottom of Lake Michigan is uniformly covered 

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

 tends out to where the water reaches a depth of about thirty-five feel. 

 Keyond this depth of water the lake bottom is composwl of a stiff, tena- 

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

 whence; in part, comes the supply which is constantly being canied to the 

 shore b.y the waves. Much of this sand is doutbtless blown iv -in the dunes 

 by south winds back over the lake, and, tailing on its surface is again 

 brought to Land. Moreover, by st(U'iiis and by ice jams in tlie sjiring all 

 Iirojec-ting points along- the lake are slowly worn down and the niateri tl 

 composing tlicm is carried out to be again returned and built up in a new 

 place. Thus mucli Of'the saiid is in cdustant circnlat imi. and the niM'essary 

 new snpiily is not as great as it seems In b(\ 



".Much gravel, consistii'g- (|f iie.bbles r.-iuging in size fnini the size of ;i 

 hen's egg to th;it of ;r small niijrhlO;- is. washed up b.\- the w;ivcs to witlii;i 

 a foot or two of the mai'gin of Ihe water. In ni.iny iilaces 11 is raked unf 



' U. S. C. S. l\I(ini>i;rii|)li .WW I 1 1, pp. 4n.''.-.f>(). " 



