That the territory comprised within the following limits be erected into a county to be called El Paso county: commencing in the main channel of the south fork of Platte River, at the south west corner of Arrappahoe county, thence easterly on the main divide between the head waters of the Arkansas and Platte Rivers twenty-five miles; thence south seventy-five miles; thence west to the summit of the Rocky Mountains; thence northwardly on the said range to the south Platte River, thence down the main channel of said stream to the place of beginning.