In The Mountain Chant: A Navajo Ceremony, Washington Matthews describes the construction of a Navajo sweat lodge:
At dawn the old man woke them and said: "Go out, my children, and build a sweat-house, and make a fire to heat stones for the bath, and build the sweat-house only as I will tell you. Make the frame of four different kinds of wood. Put kaç (juniper) in the cast, tse`isçázi (mountain mahogany) in the south, ¢estsìn (piñon) in the west, and awètsal (cliff rose) in the north; join them together at the top and cover them with any shrubs you choose. Get two small forked sticks, the length of the forearm, to pass the hot stones into the sweat-house, and one long stick to poke the stones out of the fire, and let all these sticks be such as have their bark abraded by the antlers of the deer. Take of all the plants on which the deer most like to browse and spread them on the floor of the sweat-house, that we may sit on them." So they built the lodge as he directed, and lit the fire and heated the stones.
Source: The Mountain Chant: A Navajo Ceremony. Washington Matthews. 1887.
Why must the frame consist of these four specific kinds of wood? How is their orientation important?