what event caused increased the numbers of settlers in the west, the range of commerce and contributed to the growth of cities like Chicago and Denver?

Chicago became a transportation and commercial center in 1850's. The extraordinary growth of the city in the 1850's was attributed to the rapid increase of railroads namely the Galena & Chicago Union Railroad and their effects which made transportation and trading of goods easier than the previous method of trading by steamboats. Moreover the direction of trade shifted from north-south to east-west.

The settlements in Denver increased in 1858 due to the Pike's Peak Gold Rush. This was the first historical settlement in the city of Denver.