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, Navicnla nobilis and peregrina, Cymafojjleura solea and 

 eJUptica. Here, too, are found JSltzschia siginoklea, 400 to 500 

 micra long, with its 66,000 strise to the inch, and Nitzschia palea 

 with 91,440 strise to the inch. 



The species and individuals both decrease in numbers as one 

 passes downward, /. 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 Pleuromgiua attenKatuin, which usually has 

 a length of 190 to 250 micra, was habitually 250 to 300 micra 

 long. Vyinbella 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. 



