Beginning at the mouth of the Sabine River, at the Texas-Louisiana border, thence extending west down the Texas Gulf Coast to the Mexican Border, thence west following the Mexican-Texas border to the junction of US Highway 77, thence northward following US Highway 77 to the point where it is intersected by the 31st parallel of latitude, thence east following the 31st parallel to the Texas-Louisiana border, thence southward following the Texas-Louisiana Border to the mouth of the Sabine River.