IOWA ACADEMY OF 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 splendkla, CainpulocUscus spiralis, 

 Stauroneis phoenecenteron 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 ('coiipylodiscus spiralis, a very large and pretty diatom, 

 occurs 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 above 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.— Very 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 

 4 



