142 Geology of Upper Illinois. 



abundance, however, surpassed my expectation. They first at- 

 tracted attention soon after leaving the twelve-mile house from 

 Chicago, and appeared to form a belt between a quarter and half 

 a mile in width, whose direction was north-west and south-east. 

 In crossing this belt, it was uncommon to pass many rods with- 

 out encountering a boulder. In general, they were rather more 

 than half buried in the soil. They varied in diameter, from ten 

 inches up to three feet, and belonged to the following species of 

 rocks ; granite, granitic gneiss, and trap. A few detached boul- 

 ders only were noticed between this deposit and Ottawa. Soon 

 after leaving this place, however, another band or patch of them 

 was passed. They were here scattered over the Illinois bottoms 

 so plentifully, as to prove objects of no inconsiderable annoyance 

 in the road. Two miles below this locality, likewise, a number 

 of large masses were seen. Others were occasionally met with 

 south of this point, out upon the rolling prairie, in the direction 

 of Vermilionville. What serves materially to heighten our inter- 

 est in these boulders is, the consideration that they must have 

 been transported over a distance of between two and three hun- 

 dred miles, since the southern shore of Lake Superior is the near- 

 est region affording rocks of a similar character, in situ. 



Coal Formation. 



The northern boundaiy of the coal formation in Illinois, I can- 

 not define with precision, not having been able to explore its lim- 

 its in detail. As the result however, of inquiries from intelligent 

 persons and such observations as I was permitted to make in the 

 region of the Illinois river, I am led to adopt the following as its 

 general outline : — a line commencing just north of the mouth of 

 the Kankakee and running west, across the Fox river, about eight 

 miles north of Ottawa, and the Little Vermilion, five miles north 

 of Rockwell. Whether it still continues without interruption, 

 across the country to the mouth of Rock river, a distance of about 

 one hundred miles, is uncertain. Coal is found on Green river, 

 however, at a distance of twelve miles above where Rock river 

 enters the Mississippi. It is possible, instead of thus crossing the 

 elevated country to Rock river, that the border of the coal-field 

 simply sweeps round by Princeton, ten or fifteen miles west of 

 Hennepin and Henry, and then descends the Illinois river at 

 about the same distance from its western bank. From the mouth 

 of the Kankakee, the coal passes off in a south-easterly direction 



