26 HARPER: PLANT POPULATION OF MICHIGAN 
elevations are only about nine hundred feet above the Great Lakes, 
and are near the southern edge of the region, so that there are prob- 
ably no differences in vegetation that can be ascribed to altitude 
alone. Several series of supposed ancient beaches, formed in glacial 
times when the Great Lakes were considerably higher than at pres- 
ent, have been traced by Leverett and others, but as in the case of 
the supposed marine Pleistocene terraces of Maryland and other 
Atlantic states,* these episodes of geological history seem to bear 
no obvious relation to the present vegetation, except as they may 
have locally influenced soil or topography. (In other words, the 
vegetation of an area that has been above the water say a hundred 
thousand years does not differ noticeably from that of one only 
ten thousand years old, if the soil is the same. And if the soil is 
not the same, that is a matter for the geologist to explain, not for 
the botanist.) 
Being on a peninsula not exceeding a hundred and fifty miles 
in width, this region has no large rivers. The drainage area of 
the largest, the Cheboygan, covers about sixteen hundred square 
miles. On account of the absence of rock ledges there are no 
waterfalls, but in descending a few hundred feet from their sources 
to their mouths the rivers necessarily traverse some gravelly 
rapids, which are being utilized more and more for water-power. 
The presence of many lakes and swamps and the coincidence of the 
hottest season with the season of greatest precipitation makes the 
flow of all the streams pretty steady, and on account of the pre- 
vailing sandy soil and the scarcity of cultivated fields they carry 
very little sediment. (In these respects also this region resembles 
Florida more than it does most of the intervening states.) 
Climate.—The average temperature is 41°-45° F., the January 
mean 18°-23°, and the July mean 66°-69°. The proximity of two 
of the Great Lakes presumably makes the difference between sum- 
mer and winter climate a little less than it would be otherwise. 
The average growing season or period free from killing frosts is 
125 to 145 days, the average annual snowfall about seventy inches, 
and the absolute minimum temperature about ——40° F. 
This is one of the driest parts of the eastern United States, 
having only about thirty inches of rain and melted snow annually. 
* See Geog. Review 4: 224-225. Sept. 1917. : 
