50 
IOWA ACADEMY OF SCIENCES. 
where the undertow keeps it clean, is covered with a bluish- 
black material, light of weight and very soft and yielding, com- 
monly called mud. This is everywhere underlain by a heavy, 
tenacious, gravelly, blue clay. In the west end of the lake, at 
what is called the rice beds, where the water is only about two 
feet deep, the aforesaid mud reaches a depth of eighteen feet. 
Near the center of the lake are the “moss beds,” with the 
water three feet deep and a deposit of twenty-one feet. 
About midway between the “moss beds” and the eastern 
shore the water reaches its maximum depth of fifteen feet, with 
the depth of the deposit undetermined. 
On examination this deposited material proves to be 75 per 
cent organic matter, 12} per cent fine sand and 12} per cent 
diatom frustules. On an average this would give us a bed of 
diatoms two and one-half feet thick over an area of eight or ten 
square miles. 
The number of species found- in this material will number 
sixty, among them some of the largest and most beautiful 
fresh water forms known, such as Suriraya robusta and 
biseriata, Navicula nobilis and -peregrina, CymatojMura solea and 
elli'ptica. Here, too, are found Mtzschia sigmoiclea, 400 to 500 
micra long, with its 66,000 striae to the inch, n>nd Mtzschia 2 Mlea 
with 91,440 striae to the inch. 
The species and individuals both decrease in numbers as one 
passes downward, i. e., there are more diatoms growing there 
now than at any previous time. The increase in species has 
most likely been accomplished by waterfowl carrying hither 
bits of mud from other lakes. 
One feature of the Clear Lake diatoms was very puzzling. 
What appeared to be Pleurosigma attenuatum, which usually has 
a length of 190 to 250 micra, was habitually 250 to 300 micra 
long. Cymbella ehrenbergii presented the same variation, as did 
likewise half a dozen other species. This variation toward 
larger forms came, finally, to be so common an occurrence 
that it was concluded that it perhaps was due to an extraor- 
dinary amount of silica in solution, and other very favorable 
conditions. 
Spirit Lake is located in Dickinson county, Iowa, on the 
western part of the Wisconsin drift. It resembles Clear Lake 
in most essential points. The sand beach, however, is wider, 
and it is not so rich in microscopic forms. The deposit is not 
so great, but the water somewhat deeper. 
