The county of Rock is bounded as follows: Commencing at the center of the channel of the Niobrara River, on the section line between sections twenty and twenty-one, township thirty-two, range twenty, west, of the sixth principal meridian; thence south on the said section line to the southwest corner of section thirty-three, township twenty-nine, range twenty, west; thence east to the northwest corner of section four, township twenty-eight, range twenty, west; thence south to the southwest corner of section thirty-three, township twenty-five, range twenty, west; thence east to the range line between ranges sixteen and seventeen; thence north on said range to the middle of the channel of the Niobrara River; thence up the center of the channel of said river to the place of beginning.