CHAPTER XII.

usually set down on maps and mentioned in geographical works as Lake Cayman. The Nasas is said by Dr. Wislizenius to be the Nile of the Bolson. Coming about 150 leagues from the western part of Durango, from the Sianori mountains, it runs north-westwardly and northerly towards this Pouch, and the wide and level country along the river is yearly inundated by the floods, and owes its fertility to this circumstance. The limits of the Bolson de Mapimi have never been clearly defined either geographically or politically for its immense wilderness has been neither fully explored or occupied in consequence of the danger of encountering the robber hordes by whom its recesses are infested. The northern portion is supposed to belong to the State of Chihuahua, and the southern to Durango. Nor are its general physical properties clearly known, though the common and perhaps erroneous impression in the country is that it is a low, flat, swampy country and a mere desert. The two terminating points of Dr. Wislizenius's transit through the Bolson are Mapimi, where he entered it, and El Paso, or a point between Paso and Parras, where he left it. At Mapimi, the elevation above the sea was 4,487 feet; in the valley