There is a singular geological formation in the northern part of Mexico, lying on the road between the cities of Chihuahua and Monterey, and extending northwardly from the towns and haciendas of Mapimi, San Juan, San Lorenzo and San Sebastian towards the Rio Grande, called the Bolson de Mapimi, or Pouch of Mapimi. Leaving Mapimi, the road continues about three miles to the eastern mountain chain, and then winding nearly two miles through a cañon, or gorge, it leads to a very open level valley, which is the commencement of the Bolson. Towards the right of the road, eastwardly, at the distance of from three to five miles, a steep, high mountain chain of limestone, rises precipitously, while another chain towers up to the left, at the distance of about twelve miles. Both chains gradually diverge, but especially the eastern arm, which stretches north-eastwardly and then bends to the south-west, at an angle, leaving a deep cul de sac or depression in the middle from which the country has probably derived its name. All around is an immense chapparal plain, while in the distance the Rio Nasas runs towards the north into the immense basin, and forms the large Laguna de Tlagualila, 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 of the Nasas, at San Sebastian, 3,786; at San Lorenzo, 3,816; at San Juan, 3,775; and towards the eastern edge of the Bolson, at El Paso, 3,990, and at Parras, 4,987. We perceive, therefore, that the valley of of the Nasas, which may be called the vein and centre of the Bolson has a mean elevation of 3,800 feet; and though from 500 to
