99 
cupant still persisting. The slight current, the frequent over- 
flows, and the disproportionate extent of water areas in the 
bottom-lands result from 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, Ill., near which place our plankton collec- 
tions have been made, as determined by Mr. G. 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 shghtly higher than these extend for some 
miles up the Sangamon, and for a short distance along the 
