The use of spruce bark as a building material in the Northwest and throughout the extreme northern range of the birch-bark canoe has been discussed in earlier chapters (pp. 155 to 158). In these areas, the emergency canoe was usually built of caribou skin. On the Alaskan coast seal skin may also have been used, but generally it was used for the permanent kayak-type canoe and not for a hastily built temporary craft. The caribou-skin canoe was also built as a permanent type, in either kayak form or somewhat on the model of the spruce-or birch-bark canoe of the area. However, although references to temporary craft covered with caribou skin exist in early accounts of the fur trade, there is no record of their form or details of their construction. Early in the present century some of the Indians of the Mackenzie River country built skin canoes much like the modern canvas-covered freight canoes. Also, some of these skin canoes were built so that they resembled York boats or the whaleboats of the white man. No observer has described the methods used to construct the emergency canoe of the Northwest; we do not know whether they resemble those used in the Indian bark canoe or in the Eskimo skin boat. _Retrospect_ In view of the inclusion of skin boats in this discussion of bark canoes, it may be well to emphasize again the fact that the North American Indian's method of constructing bark canoes and of temporary skin canoes was on an entirely different principle than that used by the Eskimo in building their skin boats. This is even true of the kayak-form bark canoes of the Northwest, despite their superficial similarity in design and proportions to the Eskimo skin kayak. As has been stated, the Eskimo construction required a rigid frame, with all members fastened together with lashings and pegs, the skin cover being merely the watertight envelope and not a strength member. This system of construction marks primitive skin-boat design in most parts of the world. The Indian bark construction, on the other hand, did not have a rigid frame, and all but a few of the structural members were held in place by pressure alone: the sheathing was held against the bark cover by pressure of the ribs; the stem-pieces, in most cases, were held in place by pressure of the ribs, gunwale sheering, or headboards. In fact without the bark cover in place, the greater part of the wooden structure of the bark canoe would collapse. Not only was the bark cover the fundamental basis of construction, it was to a great extent a strength member, though by clever design the loading of the bark was minimized.