harvest. Off-farm income usually consists of low pay for unskilled labor. Those who take jobs in the mines receive a slightly better wage. Forestry is an increasingly important economic factor in the region. Trade There is little evidence of much trade and commercial exchange. Between Indians and mestizos, there was some petty trading of subsistence commodities. The household is the basic production unit, but exchange of labor (e.g., for house building or harvesting activities) accompanies beer-drinking festivals similar to the tesguinadas of the Tarahumara. O'dam and Audam The great variation in elevation (from 600 meters at the deepest point in the vast Mezquital Canyon to 3,250 meters at the crown of Cerro Gordo) produces a great variation in plants and wildlife. The choices of cultivable crops are extremely limited because of the lack of water and topsoil; another determinant is the rugged terrain cut by two deep rivers, which flow southward through Nayarit into the Pacific. Deep canyons create different ecosystems and dictate the types of crops that can be grown. Pine and hardwood forests cover high plateaus. Deep valleys, with hot, dry climates and tropical flora and fauna in the lowlands alternate with the higher, temperate zones that experience heavy rainfall in the summer and frost in winter. Agriculture and pastoralism are the main economic resources, although the lumber industry has made a minor contribution since about 1980. Maize, beans, and two kinds of squash are the traditionally cultivated crops and remain the dietary staples, given that the rocky mountains and the scarcity of water leave only a trifling amount of arable land and permit little diversification. Despite the importance of maize as a dietary staple, the Southern Tepehuan do not grow sufficient quantities to feed themselves. Around the beginning of the twentieth century, it was reported that cotton was grown for ceremonial purposes, but this practice has been abandoned. Heavy on tortillas, beans, cheese, and other farm products that need no irrigation, the Tepehuan diet is fortified by a good deal of gathered foods. These include roots, wild tubers, fruits, greens, and mushrooms. The constraints of the land greatly impinge both on the economy and on patterns of settlement and migration. Along with the pines that support the lumber industry are banana, plum, and avocado trees that are native to the area, as well as the introduced apple and peach species. Also in the more tropical areas are