Jefferson (proposed Mountain state)
The proposed State of Jefferson affected the development of the State of Kansas in 1858-1859. Kansas from 1855 through 1858 had had three Constitutions proposed, all three of them had used the same borders as defined in the Kansas-Nebraska Act of May 30, 1854; this extended the Kansas Territory westward along the 40th N latitude to the Continental Divide and then south to the New Mexico Territory and then eastward from the New Mexico Territory along the 37th N latitude over to the western border of Missouri and up to the 40th latitude North.
This placed the areas of Denver (named after Kansas Governor James W. Denver) Pikes Peak, Colorado Springs and Pueblo all within Kansas borders.
However, with the discovery of gold in the Pike's Peak area, concern grew in the population of the Kansas Territory over how the gold rush and the influx of miners to the Rockies could shift the base of power from the northeastern side of Kansas, to the mountainous region in the west of the State. The mining community also began to think about forming their own State along mining interests thus creating a State of Jefferson.
Kansas was already divided and conflicted between the commercial centers and the agricultural population.
The Wyandotte Convention 
As a result of these trends, at the Wyandotte Convention which drafted the fourth and present constitution for the State of Kansas, the borders of Kansas were debated on July 16 and again on July 28. Although a few different meridians of west longitude where all proposed, the final vote chose the 102nd west meridian (25th west of Washington, D.C.) as the western most border of the State of Kansas. This worked out fine because as was noted by the "Freedom's Champion" newspaper of Atchison, Kansas, the miners in the gold fields had proposed that to be the eastern border of their State of Jefferson. Thus the western portion of the Kansas Territory would be given to the mining community for them to form a larger State of Jefferson and to prevent the influence of the miner population from upsetting the balance of power in northeast Kansas where political power had resided up to that point.
The Wyandotte Convention ended on July 29, 1859 and submitted the constitution to a vote for ratification to be held in October. Then on September 24, 1859, the proposal for the formation of the State of Jefferson was defeated. The people of Kansas ratified the Wyandotte Constitution on October 4, 1859. Then on October 24, 1859, voters instead approved the formation of the Jefferson Territory, which was superseded by the Colorado Territory on February 28, 1861.
See also 
- "The Kansas State Constitution: A Reference Guide" by Francis H Heller, ISBN 0-313-26510-0, Greenwood Press, Westport, CT, 1992
- "Kansas Territory and Its Boundary Question: 'Big Kansas' or 'Little Kansas'", by Calvin W. Gower, Kansas Historical Quarterly, Spring, 1967 (Vol. 33, No. 1), pages 1 to 12, http://www.kshs.org/p/kansas-historical-quarterly-kansas-territory-and-its-boundary-question/13180
- Freedom's Champion, September 3, 1859, as referenced in: "Kansas Territory and Its Boundary Question: 'Big Kansas' or 'Little Kansas'", by Calvin W. Gower, Kansas Historical Quarterly, Spring, 1967 (Vol. 33, No. 1), pages 1 to 12, http://www.kshs.org/p/kansas-historical-quarterly-kansas-territory-and-its-boundary-question/13180
- Wyandotte Constitution 1859, http://skyways.lib.ks.us/orgs/schs/ritchie/education/standards/7thgrade/benchmark2/Wyandotte%20Constitution%201859.pdf