752 THE JOURNAL OF GEOLOGY. 
ALTITUDES OF ROCK FLOOR AND PRESENT ILLINOIS. 
Location Dist. Miles Tower meee Rec Hoey 
Ft. In. Ft. 
Mouth of Kankakee River..... 0.0 485.3 483 
One mile below Kankakee 
ARIE Iaaetepersusieistsue nis otek eens 1.0 481.6 44.04 479 
Head of Marseilles Pool....... 11.7 477.9 3.80 ? 
Mlamseillles IDET 5450000000000 12.7 476.8 468 * 
Marseilles, below dam........ 468.8 8.59 456-468 
POOLE Ol WAPNGIS ocsc00 0000000 1.5 458.3 84.00 457 
Above Ottawa 24 mile........ 5.3 447.9 23.54 445 
Headvotsrapldsisen eee acer 6.7 444.9 5.37 440 
lEl@aGl OF OO ssoccccacocc00de 2.6 438.1 31.38 435 
Peru, at Zinc Works.......... Wo 438.12 
; 433.9 6.90 320 
Princeton, in buried channel... 270 
Bureau Junction (Bureau Creek 
Weallll sy) to- recicbiveveavars! eictensierecets 340 
Venn pim sc acct ie shel oenerereverese 13.6 438.1 ? 
432.2 1.50 380 
Putnam, two miles from river.. 340 
IPliemry A oyseeisearetater versie eters 355 
Peoria (Bigham’s artesian well) 47.4 428.9 0.84 341 
Pekin, at City artesian well.... 10.7 427.3 1.79 325 
Beardstown. Artesian wells... 63.3 418.6 1.05 345 
IMI@utle, se IOMNNONS, 565000000000 86.1 403.6 2.09 2 
Bellefontaine, Missouri. Bridge 
TOWINGIAMOM, coo o000000c 0000 17.0 402.0 + 205 
East St. Louis, Illinois. Bridge 
HOWINGIEINIOI, 550 0000 00000000 24.3 380.0 284 
The causes for the very low altitudes of the rock floor, and 
also the slight descent of the present stream below Peru, merit 
careful consideration, while the irregularities in the rate of 
descent in the portion above Peru, need a word of explanation. 
Taking these up in the reverse order we find the irregular 
descent above Peru to be plainly attributable to variation in 
resistance to erosion, presented by the stream bed. The rapids 
occur where the stream is cutting across hard rock ledges, while 
the pools occur in the softer strata, between these ledges. This 
valley, as is well known, once constituted the outlet of Lake 
Michigan and was then occupied by a stream having much 
*River raised by dam. The dam rests on rock at 468 feet A. T. 
2 River raised by dam to 438.1 feet. The dam is located at Henry, Illinois, 13% 
miles below Hennepin. The altitudes at points below Hennepin are given at the 
natural level of the river. 
