From northern Coahuilato southeastern Puebla, an airline distance of 900 miles, the Sierra Madre Oriental presents a variety of physical and cultural qualities. In general the changes are progressive: from north to south the altitude, ruggedness and humidity increase; population density is also greater to the southward. The northern section of the sierra, as far south as Monterrey, is more like the dissected edge of an arid plateau than a mountain range; mesa and basin alike fallow for want of rain. The middle section, from Monterrey to the 23d parallel (Fig. 1) has humid ranges and arable basins, but the humidity of the ranges is scarcely sufficient to water all the basins. In the southern section the highest peaks reach into the zone of permanent snow; precipitation is greater, both on account of altitude and latitude, and most of the basin land is cultivated. Thus the Middle Sierra Madre is intermediate in quality as compared with the northern and southern sections. The Middle Sierra Madre rises abruptly from a piedmont lowland at 1,000 feet to a series of long parallel ranges, varying in elevation from 6,000 to 12,000 feet. Between the ranges lie numerous basins at about 5,000 feet. Communication from basin to basin offers only minor difficulties, but this rugged range as a whole is a distinct barrier between the piedmont and the plateau. South of Monterrey for more than 150 miles no easy westward route appears. As a result the region is distinctly handicapped in shipping out its products, such as wheat and lumber, to the east. The sierra proper shows a sharp contrast with the escarpment (Fig 1): the ranges are parallel to the general trend of the mountains, whereas the spurs and canyons of the escarpment are at right angles to the sierra and slope steeply to the piedmont. The contrast with the basin-range region on the west is largely in the altitude and humidity of the ranges. In the basin-range region the basins are dominant, the ranges low and arid; in the sierra the ranges are loftier, more closely spaced and are important factors in the utilization of the basins. In short the Middle Sierra Madre Oriental is a range-basin region.