Arapahoe County.—One of the first acts of the territorial legislature of 1855 created Arapahoe county—so named for the plains tribe of Indians—and defined the boundaries as follows: "Beginning at the northeast corner of New Mexico, running thence north to the south line of Nebraska and north line of Kansas; thence along said line to the east line of Utah territory; thence along said line between Utah and Kansas territories, to where said line strikes New Mexico; thence along the line between said New Mexico and the territory of Kansas to the place of beginning."

All the territory embraced within these boundaries is now in the state of Colorado. By the act of creation Allen P. Tibbitts was appointed judge of the probate court of the county, the plan for holding court being left to his discretion, and Allen P. Tibbitts, Levi Mitchell and Jonathan Atwood were appointed commissioners to locate the county seat, which was to be known as Mountain City. One representative in the state legislature was apportioned to the county, which was attached to Marshall county for all business purposes.

In 1873 a second county of Arapahoe was created in the southwestern part of the state out of unorganized territory. Its boundaries were defined as follows: "Commencing at the intersection of the east line of range 31, west, with the north line of township 27, south; thence south along the range line to where it intersects the sixth standard parallel; thence west along the sixth standard parallel to the intersection with the east line of range 35, west; thence north along the range line to where it intersects the north line of township 27, south; thence east to the place of beginning." In 1883 Arapahoe county disappeared. its territory being included in Finney and in 1887 Haskell county was created from that part of Finney which had been established as Arapahoe in 1873.