The position of Chicago at the southwestern extremity of Lake Michigan, with prairies of the greatest fertility stretching hundreds of miles south and west, makes the city the primary food-gatherer and supply-distributor of the great Northwest, and this has been the chief cause of its growth. In September, 1833, the Pottawatomies agreed to sell their prairie homes to the United States and migrate to reservations farther West, and seven thousand of them assembled in grand council at Chicago, and sold the Government twenty millions of acres of these prairies around Lake Michigan, in Indiana, Illinois and Michigan, for $1,100,000. Thus was this fertile domain opened to settlement. In the Indian dialect, Michigan means the "great water," and it is the largest lake within the United States, being three hundred and twenty miles long and seventy broad, and having an average depth of one thousand feet, with the surface elevated five hundred and seventy-eight feet above the ocean level. On the Chicago side this extensive lake has but a narrow watershed, the Illinois River, draining the region to the westward, being formed only sixty-five miles southwest of the lake by the junction of the Kankakee and Desplaines Rivers. This narrow and very low watershed, considered in connection with the enormous capacity of the Illinois River valley, which is at a much lower level and appears as if worn by a mighty current in former times, is regarded by geologists as an evidence of the probability that the Lake Michigan waters may in past ages have found their way to that outlet and flowed through the Illinois and Mississippi Rivers to the Gulf. The diminutive bayou of the Chicago River, with its two short and tortuous branches, made Chicago the leading lake port, and thus brought trade, so that early in the race it far outstripped all its Western rivals. Every railroad of prominence sought an outlet or a feeder at Chicago, and the title of a "trunk line" was adopted for a line of rails between Chicago and the seaboard. The surrounding prairie for miles is crossed in all directions by railways, and a large part of the city and suburbs is made up of huge stations, car-yards, elevators, storehouses and cattle-pens, almost overwhelming visitors with the prodigious scale of their elaborate perplexity. The maze of railways and streets on the level surface, all crossing at grade, as it has spread over miles of prairie and grown into such enormous proportions, presents a most serious problem, with which the city and the railways are now dealing on a comprehensive plan, by which it is hoped that before long the grade-crossings will be eliminated.