The tax assessor, when ordered so to do by the board of supervisors, shall make a separate assessment of all the taxable property of the district or part of districts or territory forming a road district. In making such assessment, it shall not be necessary for him to reassess the property in the road district, but he shall copy from the assessment rolls in use at the time, or mark and separate thereon, all the property and persons in the district or parts of districts forming a road district, so that the county tax collector, in collecting the taxes levied on such road district, may easily ascertain the property and persons situated therein.