102 BOTANICAL GAZETTE [ FEBRUARY 
river's mouth. All along the Michigan coast the most extensive 
areas of active dunes are likely to be at the mouths of the rivers, 
so that dune formation is thus. seen to be regulated by the sup- 
ply of sand as well as by the relation of the coast line to the 
direction of the prevailing winds. Since the general direction 
of the Michigan coast line is north and south, and the prevailing 
winds southwest and northwest, dune formation tends to close 
up the mouths of the rivers on both sides. The result of this 
conflict between the river and air currents is seen in the forma 
tion of lakes whose entrance into Lake Michigan is constricted 
into a narrow passageway. The tendency of the wind to close 
these passageways is so great that navigation is often difficult, 
and the necessity for constant dredging and erection of wind- 
breaks is obvious. 
The dunes at the mouths of rivers furnish a rough meas 
ure for determining the relative influence of northwest aid 
while to the north the heights are inconspicuous. The course 
of the Kalamazoo river has been deflected to the northward # ' 
this point, although it is interesting to observe that the mouth® | 
now being deflected to the southward, the river filling in on tht j 
northwest bank and eroding on the southeast. At Ottawa id : 
the dunes are about equal on both sides, and less than 60 mete 1 
in height. At Grand Haven there is an immense active él# 
on the north side of the river, 66 meters high, and with # 
advancing lee slope 45 meters in height. This dune is sie 
ing the river to the southward, and attempts to stop its ae 
_are not particularly successful. The dunes at Muskego? ia 
largest on the south side of the river. At Pentwater, Ludimg” | 
