99 



cupant stil] persisting. The slight current, the frequent over- 

 flows, and the disproportionate extent of water areas in the 

 bottom-lands result fi-om this somewhat unusual ancestry. 



LOCATION. 



The latitude of the Illinois River is approximately that 

 of the Tagus, the Tiber, the Kezil Irmak, the Oxus, the Yar- 

 kand, and the Pei-Ho. Its drainage basin lies between the par- 

 allels of 39° and 43° 15' north latitude and extends from the 

 isotherm of 45" to that of 55", a belt which, in Europe, includes 

 the areas drained by the Thames, the Seine, the Loire, the 

 Rhine, the Elbe, the Oder, the Vistula, and a considerable por- 

 tion of the basins of the Black and Caspian seas, and, in Asia, 

 the basin of the Hoang-Ho and that of the Aral Sea. The 

 position of Havana, 111., near which place our plankton collec- 

 tions have been made, as determined by Mr. Gr. S. Hawkins, of 

 the U. S. Geological Survey, is 40'' 17' 37". 19 north latitude and 

 90" 03' 55 ".97 west longitude. The area tributary to the Illi- 

 nois River at Havana lies between the isotherms of 50° and 55° , 

 and is therefore comparable with the more northerly parts of 

 the regions above enumerated. 



GEOLOGICAL FEATURES OF THE ILLINOIS RIVER BASIN. 



Illinois is the lowest of the North-Central States, the aver- 

 age elevation being but 632 feet according to Leverett's compu- 

 tations from Rolfe's survey. The range in altitude is from 

 1,257 feet, at Charles Mound on the Illinois- Wisconsin line, to 

 268.58 feet, low-water mark at Cairo. Low-water mark at the 

 mouth of the Illinois is 402.56 feet above mean-tide level at the 

 Gulf of Mexico according to the figures given by Cooley ('91, p. 

 93), 404.7 feet according to Greenleaf ('87), and 402.76 feet 

 according to Rolfe, the different elevations given being based on 

 different surveys. The present bottom-lands from the mouth 

 of the river to La Salle range in elevation from 410 to 440 feet,- 

 and bottom-lands slightly higher than these extend for some 

 miles up the Sangamon, and for a short distance along the 



