Geology of Upper Illinois. 141 



face of the prairie at Plainfield, nine miles from Juliet, and very 

 abundantly also at this last place. The beds at Plainfield and 

 Juliet, however, are not rich in fossils. The rock is quite close 

 in its structure, and where acted on by the weather, of a yellow- 

 ish buff color, much resembling the lithographic stone of Sohlen- 

 hofen, in Bavaria. The quarries at Juliet, afford it in very even, 

 distinctly stratified layers, whose position is nearly horizontal, 

 their thickness being such as to render it a very valuable building 

 material. I noticed one variety of the rock at this place, which 

 had been thrown out in sinking a well, the appearance of which 

 was very analogous to that of true dolomite, (the gurhofian va- 

 riety. ) Its color is a grayish white, with a tinge of green ; 

 throughout the masses were crevices and openings, whose walls 

 were lined with transparent crystals of quartz. 



The magnesian limestone continues very abundantly in the 

 bed of the Des Plaines, below Juliet, and recurs frequently on the 

 road across the prairie to Holderman's grove, twelve miles east of 

 Ottawa ; after which, no more rock was observed until I reached 

 the bed of Fox river, just above the village of Ottawa. At this 

 point, we strike upon the coal formation. 



Of the existence of formations more recent than the magnesian 

 limestone in this region, my own observation permits me to add 

 nothing, beyond what has already been stated under the head of 

 the lake shore near Chicago. By the kindness, however, of Mr. 

 W. B. Ogden, the mayor of Chicago, and Col. Thornton, presi- 

 dent of the board of commissioners for the canal, I am enabled to 

 annex some additional particulars. The excavations for the ca- 

 nal on the wet prairie give the following superficial formations : 

 one to two feet, black vegetable mould, and two to six feet, yel- 

 low, clayey loam, resting on blue clay. On reaching the Des 

 Plaines, the sections give, in the first place, one foot of black 

 mould ; secondly, four feet, yellow sandy clay ; thirdly, one and 

 a half feet clean black sand, and lastly, twelve feet "vegetable 

 formations Avith shells."* 



The occurrence of boulders in the rolling prairie had ofteil 

 been mentioned to me, under the significant and original appel- 

 lation, bestowed upon them in this region, of " lost rocks." Their 



* From the same source, I learn that the magnesian limestone beneath these de- 

 posits often abounds with vertical fissures, filled with clay, from one inch to sev- 

 eral feet in breadth. 



