Wash. Rev. Code § 36.04.210
Lewis county shall consist of the territory bounded as follows, to wit: Beginning at the northwest corner of section eighteen, township fifteen north, range five west; thence south along the west boundary of range five west to the southwest corner of township eleven north, range five west; thence east along the south boundary of township eleven north to the summit of the Cascade mountains; thence northerly along said summit to a point due east of the head of Nisqually river; thence west to the head of the Nisqually river; thence westerly down the channel of the river to a point two miles north of the line between townships fourteen and fifteen north; thence west to the northwest corner of section twenty-six, township fifteen north, range four west; thence north two miles to the northwest corner of section fourteen, township fifteen north, range four west; thence west to place of beginning.
RCW 36.04.210