Trim any excess fat from the edges of the roast. Place roast on a sheet of heavy duty foil large enough to enclose the meat. In a small bowl, combine the green chiles (with liquid), garlic, chili powder, oregano, cumin, salt, and pepper. Mix well and spread out evenly over both sides of the meat. Enclose the meat in the foil and then wrap the foil package in a second sheet of foil to ensure the cooking liquid will not leak out during the cooking time. Place in a large baking or roasting pan.