IOWA ACADEMY OP SCIENCES. 
49 
lemnas, potomagetons, wolfia and diatoms grow, that the river 
is literally choked from bank to bank. 
In the northeast corner of the state, however, the conditions 
are altogether different. The drainage system there is any- 
thing but poorly developed, it being a part of the state not 
known to have been glaciated. Many forms of diatoms are 
found here, not common to other localities in the state. 
The Missouri river and its immediate tributaries offer but 
little that is interesting to the diatomist. But few species are 
found here, as the rapid currents and ever changing banks 
and beds of mud do not permit of their gaining a lodgment. 
The second group of localities, the springs and bogs, offer 
us a flora at once more varied and robust. Here are found the 
larger naviculse, with Suriraya sjMnclida, Gampylodiscus spiralis, 
Stauroneis p)lioenecenteron and a host of others, all with frustules 
strongly silicified. In a little bog, fed by a spring, some four 
miles from Iowa City, in a place but six feet square, ‘the spe- 
cies Gampylodiscus spiralis, a very large and pretty diatom, 
ocours in large numbers, its only known locality in the state, 
except, perhaps, one on the Des Moines river near High 
Bridge. 
The third group of localities, the lakes in that portion of the 
state covered by Wisconsin drift, presents features of more 
than usual interest. 
Clear Lake, located in Cerro Gordo county, is about six 
miles long by three wide. It belongs to the class of “kettle 
holes,” and lies on the eastern edge of the Wisconsin drift, 
occupying the highest ground in that region. The town of 
Clear Lake has an altitude aboye sea level of 1,238 feet, while 
at Mason City, nine miles east of there, the altitude is only 
1,128 feet, or 110 feet lower. On the north, west, and south 
much the same conditions prevail. 
This falling away in all directions brings about the follow- 
ing results: 
First.— Verj little surface drainage; the slope toward the 
lake being, in some places, only a few feet, while on the west 
it reaches its greatest length of about one mile. 
Second. — There is no overflow, a state of equilibrium existing 
between rainfall and evaporation. Here, then, are the con- 
ditions necessary for the deposition of a bed of organic mate- 
rial. On examination the lake showed the following conditions : 
The entire bottom, except a narrow strip along the shore 
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