In that portion of the county lying west of Rock Island the coal measures are found as “outliers “, overlaying and resting unconformably upon the Devonia and Upper Silurian limestone, as far north as the vicinity of Port Byron, where it finally terminates. The most northerly point where a workable bed of coal has been found on this side of the river, is at Rapids City, where the seam is from four to five feet thick, and overlies the Niagara limestone, with only a few feet of thickness of shales and fire clay between. Two miles east of Hampton, where coal shafts have been sunk, are good seams from four to five feet thick. The Carbon Cliff mines were the earliest worked on the west side of Rock River. For many years extensive coal operations were carried on at this point but the limited supply of coal finally became so nearly exhausted that mining here was discontinued. The triangular piece of elevated land east of the City of Rock Island, bounded by Pleasant Valley, Rock River and the Mississippi, is a mass of coal materials, resting upon a Devonian or Upper Silurian formation of underlying limestone. All that part of the county, south and east of the Mississippi and the Rock River ranges of bluffs, is underlaid by the coal measures. In every part of the county the coal measures are covered with a deep deposit of drift-clays. At Milan, Carbon Cliff, and east of the City of Rock Island, this drift clay is from forty to seventy-five feet in thickness. South of Rock River the coal measures are more regular and more extensively developed than in the northern part of the county. The coal mining industry in this county has become most important. The thickness of the coal seems to vary from three and a half to five and a half feet and is reached at a depth of from forty to one hundred and twenty feet. Coal is raised at the principal mines by steam power. The active operations in mining have greatly enhanced the value of contiguous lands, and led to the introduction of railroads as a special means of transportation.
Source: Historic Rock Island County, pub. Kramer & Company, Rock Island, Illinois, 1908